I do worry about maintance and such. I always thought the guys at the shop would care for a tank better. (maybe I am wrong)
It's not so much how the shop treats it, it's how the other renters treat it. Neither you, nor the shop, have any way of really knowing if the tank was ever emptied and allowed to collect moisture, or filled from an un-reputable source....
Maintenance costs are pretty minimal - yearly Visual, and 5-year Hydro.
It usually comes down to a few factors:
-- Costs - as above, how many dives a year determines how long it takes to recoup the tank costs.
-- Availability/Scheduling - having your own tanks means no trip to the shop right before heading out to a dive site. Get 'em filled when you have time mid-week or whenever, and you're ready to go should the urge or ability to dive happen.
-- Familiarity with gear -- knowing the history of everything, as well as knowing exactly what the characteristics of the gear are before you hit the water. With rentals, no way to know that you won't get a last minute substitution for a different kind of tank. Unless you've got records of that configuration you may spend the first dive getting weighting set for a new gear configuration.
-- Investment -- do you have the money now to invest in tanks? If it's a choice between owning tanks and not diving, or renting tanks and having $$ for the dives, seems like an easy choice. Also, tanks seems to be one of the few pieces of diving gear that actually holds value pretty well. Everything else seems to fall of a depreciation curve the first time you get it wet, while tanks seem to hold their value pretty well, especially since the prices keep going up! Heck, even if it eventually fails hydro and is no longer usable, the metal usually has some scrap value...
Personally, we decided that owning made sense since we do most of our diving at the local quarry. The Quarry is 5 minutes from the house while the LDS is 20. If we had to do 2 trips a weekend to the LDS to rent tanks, we wouldn't have gotten in the water half as much as we did this year. Instead, we fill the tanks mid-week or whenever we have a chance, and they're ready to go when we decide the time is right (weather, household chores being done, etc...)