Play a DPS because you want to be the one that excels at damage output which makes the groups runs go faster. While your characterization of DPS isn't completely wrong, you left out the other side which is that it's incredibly noticeable, and even commendable, when you run across a DPS player that does actually know what they're doing. At least for those of us who pay attention to such things.

As for which DPS to play, that's a harder question. You need to consider the aesthetic/playstyle, if it suits you or not, and further determine which sub-role you'd like to participate in. Samurai is a high damage dealer. Dancer is a low damage dealer but does a lot to support the groups damage. Other jobs are in between, so you need to decide if you'd rather be more of a support DPS or a strictly damage dealing DPS. From there do you prefer melee or ranged, ie do you move around a lot (melee or ranged physical can attack while moving/dodging) or can you manage to work around cast times which limit mobility (casters).