I think an Aztec or Apache would be great for east coast. Rockies? I'd rather have turbos. I'd look at single engine altitude limits before buying a twin. Sure, 200fpm descent beats 500, or maybe more accurately 8000 max altitude or whatever is better than nothing, but if you're going to go to all the trouble of buying a twin, you might as well get one that will perform OK single engine for your trip.
On the other hand, if you keep making the next step toward performance, someone is going to suggest King Air...