I'm inclined to say ... both.
-- Pick a day in the middle of the week to do intervals.
-- Do your long distance riding on Saturday and Sunday.
-- Rest a couple days a week
-- Ride a medium distance on whatever's left in the week.
The intervals will help you with things like getting to the tops of hills comfortably. In fact, once I'm off the trainer and out on the road, I switch from intervals to hill repeats.
The long distances on the weekends get you used to putting lots of time in the saddle. That's where you work out fit issues, nutrition issues, etc., as well as just getting used to being out there a long time.
The medium distances improve your fitness level, help you lose weight if you need to, help you build some strength, etc.