It depends on how often you want to keep upgrading. If you're serious about making good mic tapes you may outgrow the 391s pretty quickly (just my opinion - no bashers please!) You do get what you pay for when it comes to mics. If you want something you'll be happy with for a very long time, you need to spend the money on the front end. Good mics will do just OK with a cheap pre, and great with a HQ one of course. Cheap mics will do just OK with a cheap pre, and just OK with a HQ one.
Just my opinion of course. Don't know what that's worth though.
Dirk