My wife and I have the 7 cm Downmats (regular size) with integrated pump. Before that we had 5 cm Thermarests (also regular size), and these were no longer comfortable enough fo rmy ageing bones, and relatively heavy (1200 grams) and huge to pack. The downmats are much more comfortable for me, lighter (about 900 grams), more compact (they fit inside a rear pannier), and significantly warmer (we like camping early in Spring or late in the Autumn). Finding the right pressure for a heavier person like myself is a bit hard. Too little and your hips touch the ground, too much and the mattress is too hard. For a heavy person it is a fine line, and maybe the 9 cm would be easier (but also heavier and bulkier).
Unless you are into really cold weather camping I think the R value is unnecessarily high. The Exped Synmat will do fine for the kind of temperatures that you encounter as a cyclist, I believe. It is a bit cheaper, a little bit lighter, and a bit more compact. There also seem to be fewer reports of leaks than for the Downmats. The new Synmat Basic lacks the integrated pump (you inflate it with your mouth), and has a simpler construction. As a result, it is much cheaper, much lighter (660 grams), and quite a bit more compact. It is also a little bit narrower. Its temperature rating is still some 10 degrees below freezing, which even if you take that with a pinch of salt will be fine for most cyclists. It is only at Alpine altitudes that you get the combination of rideable temperatures at day time and really low temperatures at night. So I personally think that this Synmat Basic now represents the perfect compromise for most cycle tourists.
If you are only camping at summer temperatures, there is also the new Thermarest Neoair. It is the lightest of these thick insulated (lightly in this case) air mattresses at 410 grams, and the most compact (it is absolutely tiny).
Willem