As I remember from the late-PSX-era, Tales of Destiny was tough to find anywhere, and "Tales of Destiny 2" was abundant and cheap.
After a quick check of eBay, Tales of Destiny seems to be reasonably-available, and goes for about $25-50, when complete. "Tales of Destiny 2" seems much more abundant, but is strangely going for around $45-70 (when complete, and not including the Player's Guide, which has also gone up in value).
I'm guessing that there's increased interest in the Tales series right now because of Symphonia, which is driving up the price, and "Tales of Destiny 2" is getting favored over Tales of Destiny just because of the clearly superior cover art.
Oh yeah and, I don't know if I made it clear in my other post, but the main Phantasia, Destiny, and Eternia lines have absolutely no relation to each other, besides being made by mostly the same group of people, except for the role of "main artist", which has had a different "famous manga artist" for each series (Kosuke Fujishima is possibly the best manga artist, ever), and they all share the same "linear battle system" (except Symphonia, which invented the 3D "tri-linear battle system").