So you are looking to brew something "lighter" with Columbus and Simcoe? Have you used these hops before?
They are both pretty aggressive, though I suppose if you use restraint they could be less in-your-face. I recall recently listening to a Brewing Network podcast where the guys from Anchor were interviewed. The head brewer Mark Carpenter commented on how lovely a hop Citra can be, even (or maybe especially) when it's used in lesser amounts to showcase its subtleties, i.e. in Anchor Brekle's Brown.
Although I am very partial to the qualities of noble and noble-type hops myself, it seems these days that the old rule of low alpha hops being generally superior for refined flavor is being thrown out the window. I say if you want to brew a light/session ale with restrained usage of Simcoe and Columbus, go for it! Sounds like a fun (and different) kind of beer. The new craft brewers in England are brewing exactly these types of American-style session ales that are meant to display aggressive American hop character, but of course they aren't as hop-forward as the American beers!
Both of these hops are pretty dank and resinous. Simcoe in particular is sometimes known for that 'cat pee' character when it's used in higher amounts. I haven't used it in high amounts for just that reason. Seems like narvin's advice is pretty spot-on; Columbus is good as a bitter hop (and also late), and Simcoe is better left to late/dry-hop additions.