Depending on what kind of Hero you use, Towers are normally more useful to speed level.
I have a Countess (level 70) and I use it for both. The hybrid way is something you want to start with, to get more along with the maps you are playing. When you get enough mana and a decent level, you can respec your Hero to put everything again into Tower Stats. In that way you can use a Tower Hero to level another character, which you could use for DPS.
If you are playing offline only, then you'll have to try some stats. Because I do not have a lot of experience in soloing the campaign..
Good luck! ;)