Moulds definitely is the better option. Price has really only had one really good year, and he did it as the # 2 WR. Now that he is a #1 he will face what Moulds has already has. Moulds is bigger and stronger, and can fight off the double teams. Price won't be able to. He's a quick, route running WR. He'll be a deep threat if he gets by the D, but I'm laying off him this year unless I can get him later than normal. I'd take Horn, Boston, Holt, several others from that group of more proven receivers. They've either been the #1 and succeeded, or have a complementary WR to take some heat off.
Moulds has had the tendency to follow a good year with a bad year, but it's not as simple as saying 'Moulds will have his bad year this year - so take Price'. Moulds is still a young, improving WR. He is not 'destined' to alternate good years and bad his entire career. That may happen again, since he does lose Price. I think that hurts Price more than Moulds. WR's switching teams can struggle the first year, and Vick only threw for 16 TD's last year. Lots of potential in Atlanta for a big year on both ends, but it's far from a given.
What concerns me more about Moulds is his lack of the 'big game' - the multi-TD game, and 100+ yards. I believe he has one mutli-TD game in his career, and it came last year. I think he'll be dependable and put up good numbers this year, but he doesn't carry your team in a week like Owens or Harrison can. Even the lower tier WR's have big games once in awhile. That trend concerns me more than his good year bad year trend.