Welcome to the folding world.
The main different between M6L, M6R-Plus & M3L
M3L = M type handlebar, 3 speed (Sturmy Archard Hub) L= no rack or lighter about 24 lbs. Price $830
M6L = M type handlebar, 6 speed (SRAM Hub) no rack and just a few gram more than the M3L. Price $110 more than the M3L it is no longer a standard model.
M6R-Plus = M type handlebar, 6 speed (SRAM Hub) Rack and dynamo light system. The rack and dynamo system add about 2 lbs more to the bike so, about 26 lbs.
If you are planning to purchase this bike I would recommend upgrading the tires to the Schwalbe Marathon usually $20 if you custom order the bike. Also, the Dual pivot back brakes. Which is $10 if you custom order the bike. I would also add the new E-Z wheel set as well since they are only $25 custom or $40 after market.
I have been a folding bike dealer in NYC for the past 3 years and also a heavy user of the Brompton both in terms of daily city commuting as well as long distance touring. I just came back from a month long bike trip in Hawaii traveling from Island to island with my M6R.
There are many folding bikes out in the market and they all offer different things. But stay focus as to what your need are. The following is my order of important on a folding bike.
1) The speed it takes to fold and unfold
2) The Size after folding
3) The engineering (will the bike stay folded)
4) The Ride
5) The weight
6) The Price
Once you know what you are doing the Brompton can be folded in less than 10 sec.
The Size is (10.5 x 22 x22) Do keep in mind a few inches more determines if you get into the building or not! I can pick up my Brompton with one hand and shake it and nothing will fall out. (try this with any other folding bike). The chain is in the center of the bike after folding to avoid grease getting on others. The ride is OK, not a racing bike type of ride like a Bike Friday or a Mountain bike type of ride like the birdy. But much like a Schwinn or a regular city bike. The weight is around 24 to 26 lbs an average weight folding bike. The price is not the cheapest but definitely not the most expensive like the top of the line Moulton that is about $5000.
(1) In term of light weight Brompton do offer the Titanium model but it only save you 3.5 lbs and cost you about $700 more. Unless you have a medical issue or have lots of money. Maybe it is easier to loose 3.5 lbs
(2) If you do some recreational riding outside of the city I would recommend the 6 speed model as it really offer you better range of gears for rolling hills and hills in general. It is now very expensive to do the up grade from M3L to M6L since they now use different hubs.
(3) Durable is a great Question. Most of my customers keep a Brompton for 8 to 10+ years. It is very difficult to find a 2nd hand Brompton. Brompton offer a 2 year warranty on all the non-wearable parts and 5 year warranty on the Frame and they are very good about this. Most of people I talk to about Dahon tell me after about a year or so the frame starts to make creaking noise from the joints. So, do keep that in mind.
Finally a standard Brompton will fit a rider from 4’7” to 6 ft with inseam no more than 32” if you are taller than 6 ft or have inseam greater than 32 inches an extended post or telescopic is recommended.
I am a dealer for the Brompton, Bike Friday and Mobiky I used to sell the birdy but the customer service in the US is just too poor so I discontinue it. Feel free to ask more question if you like. As to the negatives of Brompton? If you are a heavy rider or are planning to carry lots of weights. I would look into stronger spokes for the back wheel.
If you total weigh is around 200lbs. The limit on a Brompton is 245 lbs
Good luck with your search www.bfold.com