HM:FoMT is the best HM game to date. It was a grind fest. That's the fun of Harvest Moon. Rushing around all day but never getting everything you want to done. One look at HM:AWL will show you that giving players enough time to do everything they want quickly makes the game's shortcomings apparent and the game becomes boring fast.
All of this said I haven't played Harvest Moon DS. Although from the reviews I've read I'd say you'd be better off buying HM:FoMT.