Both plaid and oxford are o.k: the trick is knowing your body type and your fit. For example, you can't go wrong with a plain oxford shirt because it's considered an 'essential' item because it's basic and goes with any article of clothing.
As far as plaid shirts go, I would say watch the brand for quality and style. For example, at places like PacSun and Zumiez, the plaid shirt variety includes some really gaudy colors and plaid patterns that might really put you off (to be honest the Buffalo Plaid is THE worst in my opinion)...
personally as far as plaid is concerned, I think the styles found at places like Abercrombie, Hollister and maybe even J.Crew are clean, cut and stylish, as well as classic and you cannot go wrong with them(:
I like both. Variety is always nice. Wear plaid one day and a plain polo the next day.
I really like billabong and quicksliver button down shirts. I think they are nice and make a guy look really fresh and cleaned up. I live near the beach, though, so its what many guys around here wear whenthey go out.