First of all, to be buying an iPad for your boyfriend you must be LOADED, lol.
It really depends how much music and apps you think he would use. I personally don't use my iPad for music at all - I have an iPod classic that fits my 80GB of music, with about 70 free GB to fill up if I so desire. I have quite a few apps but they only take up 3.5GB. I have a lot of iPad friendly movies but I only put a couple on at a time. For me, the 16GB works just fine. I still have 6GB free on it right now. But if you really think he would use much more space than I do, go with the 32 or 64. Apple.com lists the prices, but for WiFi only models, 16GB are $499, 32GB are $599 and 64GB are $699.
In terms of specs.. all iPads are the same, they just have more storage in some models.
The price is always the same except for refurbished models or in the slim chance someone has them on sale. So it doesn't matter if you buy it at an Apple store or at a Best Buy. I personally bought mine at the Apple store, which I will also do with my iPhone 4S (in December when I can get one). Just something about that store. :ipad-back:
Hope this helps.