If you want to fly fish, spring chinook are a real challenge. Much harder to get, IMHO, than steelhead. Most spring chinook fishing is done from boats on the larger rivers by people dangling herring and spinners and such.
If you go with a guide for steelhead on day one, you will know where to go and what to use on day two. Personally, I'd go for the steelhead, unless water conditions are so bad that it's a hopeless proposition. Trout fishing on the Deschutes can be very productive this time of year, as the fish start to key on the salmon fly nymphs. On warm afternoons there are often hatchs of various Mays (Blue Winged Olive) that can provide some surface action.