It all depends on where you see your career going. If you see yourself staying in and wanting to stay on the front line, then QWI is the way ahead. Invariably you will get picked up and also involved with the more esoteric tactical stuff which can be very rewarding.
If, on the other hand, you decide to go QFI you can expect to go back to the training world before returning to the front line. It's also a reasonable qual to stick on a CV, but don't expect it to open too many doors in the airline world. It can be v. rewarding particularly when training youngsters. It can also be v frustrating when trying to retrain hairy old nose gunners!! Either way ..... get a "Q" tick rather than a TI if you can.