I found the 126.96.36.199 to be the most stable. But, i don't really see any difference between .107 and .108. I can't use .109 cause i don't have an edited .alx file but, i'd go with .108 and see how it works for you. if bad, then try .109 or .107, know what i mean. But it's good to have the latest software cause they are always doing upgrades and improvements to the software.
And when it comes to upgrading...i've never had a problem as long as i carefully followed the instructions these guys laid out in the "instructional manuals" and so on.
What ever you do just don't do any sudden things when you're updating your software. Meaning that if you see the hourglass spinning for 30 minutes...let it spin for a day if you have to. Im exaggerating of course but after the update is complete, the hourglass on ur BB will spin anywhere from 15 to 45 minutes. And the desktop software will wait for the BB to complete what ever it's doing during the spinning, and will then continue with the update. Also, don't be encoding video or using ur computer heavily when ur updating. I recommend you start the update and go for a walk or something and then come back in about an hour, and the update will be done.
Also make sure you have the most recent version of the Desktop Manager software for your PC and, that the software to be updated is the appropriate version to ur BB. ie, don't try to place the 8120 software onto a 8100. And secondly, if your BB is locked to a particular carrier, then just update the carrier' latest software and not the "global" latest software version.
I'm not sure what your level of experience is so don't take me the wrong way if i'm going over ground you are already familiar with.