I have a unibody 2.0 GHz MacBook right now. I only have it because my rev B MacBook Air was stolen a few weeks ago. I absolutely would prefer a rev B MBA to a uMB.
The differences are extremely minor as far as components are concerned.
The CPU in the MBA is actually a really nice Penryn 45 NM SSF Core 2 Duo. While the clock speeds are a little lower, the CPU in the MBA does have 6 MB L2 cache. I would say the 1.86 in MBA and 2 GHz in MB are very similar.
The MBA has the same GPU slightly downclocked, but I would doubt you would see a difference. The RAM and BUS speeds are the same. The MB ships with 2 GB of RAM but is upgradeable to 4 GB, while the MBA is not upgradeable from 2 GB.
The MBA uses an external drive, which can be a pain if you use it often. For me, I never used it except the day I installed the software. If you have a PC or other Mac available, it's simple to install disks via a remote drive.
Perhaps the most important differences come with the drive. The MBA uses a 1.8" drive with a proprietary cable. This means you cannot easily upgrade the drive. The drive in the MBA doesn't have as much file space as the drive in the MB. However, the drive in the MBA can be a big advantage, in terms of speed, if you go with the SSD. The high end MBA with SSD makes the MBA boot in half the time of a MB. It makes the MBA feel faster than a MBP for common tasks. Apps open instantly. Large files are a breeze. The SSD is the quickest way to make a Mac faster. It is expensive, but I believe it makes the MBA experience go from fun to incredible. It makes the MBA go from average speed to insanely fast. If you go with the HDD, the MBA will feel slightly slower than the MB (4200rpm HDD in MBA vs 5400rpm HDD in MB).
Both are built with a wonderful aluminum brick frame/case.
The display in the MBA is actually MUCH NICER than the display in the MB. The MBA feels so much lighter than the MB. Carrying around the MBA is like carrying a few magazines in a bag. Even though it only weighs 1.5 lbs less, it feels like it's a fraction of the weight of the MB.
I would say if you can afford the high end MBA, it will be worth trading. If you cannot afford the high end MBA, you should wait until you can before switching.
Lastly, may be worth waiting a month to see what happens. There are some rumors going around that the Mac notebooks will be updated in June. However, it may just be the MB and MBP getting updated in June. I am waiting to see what happens in June. If a rev C MBA is released in June, I will buy that. If nothing happens, I will buy a rev B MBA just like the one that was stolen from me. I just don't see an advantage to buy now, as the MBA is nearing seven months of age, and the price is the same as the day it was released. It's like paying full price for six month old technology. At the same time, I cannot wait more than a month to get rid of this MB, as I really miss my MBA with SSD.
Good luck with your decision.