I have a 97 rallye. It is a 640 and the same engine as in the Adv. The advantages: are the rear gas tanks at about 5 Gals.(20ltrs.). I heve never used them as the 7&1/2 gals(29ltrs) up front and 50MPG(80KMpg)give me plenty of range. The strengthening of the rear subframe is great. The fairing headlight and its mounting system is solid for Off-Highway use. The plastic is nylon or similer vs. plastic on the Adv. By that I mean it is more dirt bike like nylon. I have a friend with a 640 Adv. and they feel the same. Ours have the same gearing. He thinks mine vibrates less, I feel his vibrates less. They do vibrate! But that engine seems bullitproof. I have 6000 miles on mine, mostly off road use, and it never skips a beat, not one problem.
Disadvantages: headllights are dirt bike like vs. what seems to be more road oriented on the adv. No turn signals. No rear rack, and with the rear tanks it is difficult to fit panniers, but it can be done. No speedo or odometer, warning lights etc. I do have the ICO rally computer, but not the same. The seat will kill you but that is fixable. I wouldn't trade mine for anything but would I rather have an adventure 640? Probably. I do have a 950S, which solved that problem. Hope this helps. If mostly used for dirt the Rally might be better, but the adv. will get the job done as well, and it is better for the tarmac. Hope this helps.