Some of the other folks in here know quite a bit more about folding bikes than I do, or have specific experience wrangling them onto a boat. With that caveat, and based on your requirements, I'll say what I would get for myself:
1. A Brompton. Since it's a backup bike, I'd choose simplicity over features, but would get some key upgrades.
-Starting with a Brompton C3;
-add mudguards
-add a front carrier block AND a front cloth folding basket
-
in addition to the front basket or
instead of itadd a brooks saddle AND a dependable saddlebag (maybe Carradice)
-add a telescopic seat tube (unless you and the wife are both under 6'0", then ignore this)
-add a bag/box/suitcase to keep it in. Rather than spending lots of money on a brompton bag, I'd craft my own from scrap materials. Mid-term elections mean that there are going to be a lot of leftover election signs in a few weeks. Many of these are made from corrogated plastic (coroplast) and can be glued or sewn together into a dirt-cheap, sturdy box custom-shaped to your bike's smallest folded dimensions.
Pros:
Folds and unfolds quickly
Gets amazingly small
Solid reputation for company/quality.... bromptons get kept. People don't seem too eager to sell them off.
Cons:
Difficult to get the Brompton that *you* want without some work and *ahem* money thrown towards accessories, which are arguably overpriced and available only from Brompton dealers.
Spare parts? Standardized parts? Haven't owned a Brompton myself, but have read some grumbles in here about Bromptons.
2. A Swift folder, also customized in some particulars:
-add MKS EZY quick-release platform pedals, or alternately folding pedals.
-add quick-release fenders
-add brooks saddle AND a saddlebag
-possibly add a handlebar bag
-If gears are needed, stick with internal hub (SA 8spd and Nexus8 have been getting a lot of compliments in here); otherwise, single-speed.
-Add Big Apple tires. I personally use high-pressure slicks, but the Apples have been getting glowing praise from folks in here.... especially since they're going to probably sit on the bike deflated for long periods of time, I think they'd serve well for something like this.
Pros:
SOLID construction
Mostly standard parts. Spare parts and upgrades can be had from nearly any bike shop.
The bike can be taken in many directions, depending on how you want to upgrade it. Lightweight, drop bars, commuter, track bike, fixed-gear.... and you don't have to go back to the manufacturer for the upgrades or accessories.
Not cheap either, but the bike seems to adapt more easily than a Brompton to a variety of riding styles and with the components that you choose, based on brand/quality/price/whatever.
Cons:
Well, it doesn't get *as* small as a Brompton
To fold it down as far as it'll go, it's more a disassembly than a folding process. It takes a minute or two, and you wind up with a jumble of parts.... stem/handlebars dangle from the cables, the front wheel is just loose... you'd want a bag or box or bungie cables to hold it together. It's not as "neat" as a Brompton.
In all honesty, I'd lean more towards the Swift. The Brompton is very good at getting very small, and pretty quick. The Swift on the other hand, will get somewhat small, very quickly.... and get smaller with a minute or two of jiggling (releasing the qr stem and taking of the qr front wheel. The Swift isn't as impressively small as the Brompton when it's folded, but it does get super thin, even if you don't fold it up all the way (just turn the handlebars sideways). The QR-pedals or folding pedals will get it even thinner. I'd recommend bagging/boxing it when it's stored between uses, but it can be leaned inside a closet without too much trouble. The bottom bracket is at a fixed angle from the rear hub; even when being folded, they move together, so you don't need a chain tensioner to keep the chain from falling off when it gets folded. The Swift design is spare, almost austere. It has about as few moving/pivoting parts that a folding bike can have while still compacting down.
Bromptons are in many ways like Apple computers. They're stylish, a bit geeky, and they'll serve you well, but you have to be willing to do things "The Brompton Way", just like "The Apple Way". You'll pay a premium for the hardware, but find it very user-friendly. Upgrade paths will often be limited by the design.
In either case, I'd shie away from a rear rack on either one. I specifically bought my DownTube because of its rear rack (as opposed to the full-suspension model), but rarely if ever use it. I tend to find three useage scenarios with rear racks on *most* folding bikes, none of which I find acceptable:
1. A rear rack built onto the bike. Assuming that the bike has 20" or 16" wheels, I find it very hard to put *any* useful load on the rear rack, because anything that hangs over the edge of the rack tends to interfere with pedalling. The smaller wheels are so much closer to the ground that my heels scrape against whatever's loaded back there.
2. A rack that attaches to the seatpost. Most of these are rated for rather conservative amounts of weight....again, enough to carry 3 books or a jacket, but not to go grocery shopping. These racks can also interfere with the fold of the bike.
3. A permanent or QR rack which "stands up" from the rear hub. These tend to sit at closer to the height of a regular rack, but have to compromise either by being permanently mounted to the bike (interferes with fold) or having no place to put it when you fold/disassemble the bike. This may work if you take it off at home and stash it in your garage somewhere, but if you're riding the bike to work, and fold it so that you can carry it inside the building.....
This is why I'm leaning more and more towards saddlebags. I've ordered my first Carradice saddlebag and plan to use this for my luggage from now on. One thing that many folding bikes have in common is a seatpost that sticks far, far up from the frame or tires of the bike. Even a large saddlebag can be fit in the crook behind the seat, without having to worry about tire-scraping. I used to have a hybrid bike to which I tacked on a rear rack and 2 Wald folding baskets (now stolen
; hence the switch to folding bikes). I'd love to do something similar to this on my folding bike, but this is one area I'm having to compromise in. I would predict that for someone used to riding an Xtracycle, the deficiencies of racks would be glaring. If you are willing to compromise, however, I think that you can get in a decent amount of groceries by using a large saddlebag and maybe also a handlebar bag.
Good luck with narrowing down your selection, whatever you choose. We'd love to see pics and have you and the wife chime in with your opinions when you do eventually make a purchase.