Both have good points, RO/DI will be cheaper than buying up distilled, if the tap water you're using for the ro/di isn't really really bad. I spent over $100 on my RO/DI unit, now it sit in the corner unused since my source water here is so bad, new filters every couple months isn't worth it, I buy up DI water from a LFS here instead. It's cheaper for me. I'd recommend getting a TDS meter either way first. That way you can check the tap and make sure it's not going to be a waste of money to get an ro/di unit. If you wind up having to keep purchasing water, you'll have a TDS meter to test it and make sure you're not getting constantly changing water from the same source. (buying water is only more effective if the retailer is changing their cartridges properly, I havn't bought "bad" water from LFS yet, but i could see it happening)
Oh, and RO is much better than Distilled. And DI is even better than RO. And if the LFS is using just tap, it's probably not so much from the idea that it's just as good, but more for the reason that it's cheaper on the store. That or they think they know more than they do, which I run across in alot of LFS.