I asked this question in April, 2007. I was unable to find concrete answers, eventually opted for a Mini, and have been pleased with the decision. The following is a combination of fact, heresay, opinion, and speculation:
The Curve --which I have not ridden, nor even seen-- offered a number of attractive features that the Mini lacked, namely:
--Better tires (Schwalbe Big Apples)
--Pump built into the seatpost
--Magnetic clippies to hold the frame together
--Fenders
--Rack
But in practice I've heard the rack is so useless that it is no longer included; the fenders are no longer included; the magnetic clippies are unreliable; and the pump has problems as well. Can't beat the tires, though!
The Mini, in contrast, has a couple advantages over the Curve:
--It has real rear suspension, which cannot be retrofitted to the Curve.
--It has a better hub, which would be a major expense to retrofit one to the Curve.
Whether rear suspension is a significant improvement --or any improvement at all-- is a matter of opinion. I have modified mine by the Jur method (there's a thread on that) and now I like it just fine, but it did not effect my decision last year, and were I to do it again, it still wouldn't. The 8-speed hub, however, is a major improvement over the 3-speed.
The crucial question is: which frame is better? Which rides better? Which is stronger? And I don't know the answer; I'd love to hear from someone who has really compared the two. I have heard that owing to the frame design you cannot fit a larger chain ring to the Curve, which effectively limits the gearing; the Mini does not have this limitation.
Bottom line? I don't know! But....
If there's anyone with a Curve, either model, who would like to get together with me for a ride in Central Park NYC, I'd be interested in doing an overall comparison between that and the Mini. I'll bring my Mini, you bring your Curve, and we'll go for a ride. Trade bikes for a while, try the hills, try the flats, try keeping up with the roadies... and see what gives. Anyone?
I have never ridden the Curve. But one can get a front mount bag -- on the head tube -- which is a real bonus on the Merc/Brompton. I suspect that it works well with the Curve too.
I don't recall negative statements regarding the rear rack. But then again, I was not looking. I recall that someone got a top mounted rack on the rear and could fold the bike as is.
I think that the Mini is a great machine except that it isn't particularly good at carrying stuff since most mounted racks would interfere with the fold. You can get a nashbar front rack with the 15-pound capacity on the front fork and fold the bike.
I went with the Mini for the 8 speed hub.