They're both great.
I am in the minority, but I found Rogue Galaxy to be an average title. There is a lot to do and it can easily take you into the 100+ hour mark. My complaints, however, are minor. I hated the villain (not in the guy-you-love-to-hate kinda way), found some of the dialog a little off (but the voice actors were spot on for the most part), and couldn't stand the enemies that were only vulnerable to jump-attacks. I beat the game in just over 40.
I slightly prefer Odin Sphere. The art direction was great, but it got a little too heavy plot-wise at times. It's very serious throughout. Contrary to a lot of reviews, I didn't find the game all that difficult. You will die - it doesn't take much - and there are a few boss fights that are a little frustrating, but it's not a hard game. I I beat Odin Sphere with every possible ending in about 30 hours.
As far as cost is concerned, Odin Sphere is an Atlus game, but I don't see it as being one that will go up in price over time. My guess is that it will be similar to Rule of Rose where it will be semi-rare, but can be picked up for around $25. If you've got a Fred Meyer or Smith's Marketplace, you can pick it up for $29.97 + whatever % off clearance they currently have. Rogue Galaxy is destined to be around for some time.