Consider this. If you anticipate crashing, as most riders occasionally do if they ride at all aggressively, you definately want to trailer a bike to the riding area. It's bad enough to have to walk back to (hitch a ride back to) the staging area, it's unthinkable to be stranded without any way home. With a trailer you can carry more tools, spare parts, and gas and not worry too much about being stranded. I bought one of these 4' X 8' do it yourself foldup thingys and it works great. It folds up to about 6' high by 5' wide by 18" deep behind my apartment.
Having both a '91 DR 650S and '84 RM 250E, I recommend that you get a strictly dirt bike and a trailer. You can find (at least in Southern California) hundreds of older RM's, KX's, RMX's, whatever for less than $1000 and they're a lot more fun in the dirt than a dual purpose. (Ok, ok I know the DR 650 isn't really meant to be an aggressive dirt machine, still though...) Overall I'd bet that you'd spend less on a strictly dirt bike and trailer than on a dual purpose. Of course if you don't have a tow vehicle this doesn't help. If you don't want to buy a trailer, U-Haul can rent you one for about $10/day. They usually rent pickups too.
You might make note of the thread running about off-road riding gear too.
Glad to have another joining the ranks!
Eric Hobson --Standard disclaimer
San Pedro, CA