I have had three airs, the Rev A 1.6/HDD, Rev. A 1.80 w/SSD and currently own Rev. B 1.86/128gb SSD. All have been wonderful although there are limitations. The memory is soldered on the motherboard so the max is 2GB. Speed wise, using MSR tools my Air is almost always in a lower EIST state 600-800mhz, and spikes to 1.6 or 1.86 only when needed. There is far more of speed difference between SSD and HDD than there is between the GHZ of the processor. Mainly because the CPU rarely if ever sustains the 1.86ghz speed. When I had both Rev A's, I could NOT even sustain 1.80ghz for more than a few seconds, thermal throttling kicked in and reduced the speed to 1.6ghz. Pretty effen lame.
The Rev B's are much better in not throttling as quickly, but cooling is still an issue. As for the HDD/SSD debate, go with the SSD, the Air's use 1.8" drives and don't use standard SATA connectors so a user-upgrade on the HDD to SSD is difficult. Heck the Rev A used a PATA IDE "ZIF" interface, Rev B's use a SATA "LIF" interface, so trying to even find an SSD in 1.8" form factor with a LIF instead of SATA is tough.
I haven't looked into the new Rev C's, but it seems identical other than the speed bump.
