Like Lead Sled said (say that three times fast!), it's basically a 340 - however I can't see having a cabin class twin without pressurization. If you've been doing the same missions in the Seneca without any problem, maybe it's no big deal for you, but especially in the Summer out here in Texas - I'd be miserable if I had to deal with flying low all the time. The cumulous commonly gets up into the teens, and it's just much nicer to be able to fly above it all. Looking at prices, it doesn't seem that the 340 commands much of a premium over the 335.
That said, I've always enjoyed flying the 340. It handles well, lands easily, the fuel system is reasonably simple (just remember that when running on the aux tanks, it's pumping one gallon of fuel to the mains for every gallon you burn!), and the only thing that'll make it any harder to deal with than the Seneca is that you have to be much more careful with the engines. You just have to plan a little more with your descents.
The 340 I flew was a RAM IV model, and had an increased gross weight - giving it a useful load of just above 1800 pounds. So using your 500 mile example (with reserves), you'd have about a thousand pounds to play with between people and bags. I burned about 21.5 gallons per side, giving me about 205 knots in the mid teens.
