I haven't tried the 8100, but I have the V and I have used my brother's 815 a lot. The 815 does have really good reception and it sounds amazing as far as phonecalls go. The V also has good reception, but not as good as the 815. The Only thing I don't like about the 815 is the outer screen isn't that great and the build quality is suspect. Phone feels floppy and slightly creaky. The V has held together really well (got it when it first came out). As far as size goes, there is really not that much of a size difference between the v and the 815 when closed (if you take the antenna on the 815 into account). The V is heavier though.
Lastly, I really like all the things you can do on the v now, especially vz navigator. If you haven't used it, go to a store and demo it. You can tell it to find a place by name, or you can tell it that you want to go to a chinese restaraunt (for example). It will find the closest restaurants to your present location and you select one of them and go! It's really amazing.