I'd go with the RM250. I just returned from riding CRF450's to a highly mod'd 03 CR250 & love it. No more stalling coughing bogging that'll make you crash. Maintenance is 100% easier, as is carb tuning. Handling is light and easier. My bike is PC modified & it's a bit faster than a stock crf450, but with the previous benefits noted.
It is totally a matter of personal preference IMHO. At the very highest levels of competition (i.e. outdoor pro-nationals) the 450 might have a slight edge, but at the amature level I'd give the edge to the 250 as its lighter, easier to maintain and imo more fun to ride.
If you race competivley and are a true B rider or above. I would go with the RMZ450. If you are a local guy who does his own maintence and races once in a while and just likes to ride without checking valves and cam tensioners, I would go with the RM250. Also if your ride more supercross orientated track the RM250 is the better choice.