How to select a tour operator?

My partner and I are looking to do a 6/7 day Safari starting and ending in Arusha at the start of October. We're quite limited by our budget ($1200pp) but we've shortlisted about 5 tour operators. They all seem to have very similar itineraries and similar prices. Day 1: Arusha - Tarangire Day 2: Tarangire - Serengeti Day 3: Serengeti Day 4: Serengeti - Ngorongoro Day 5: Ngorongoro - Lake Manyara Day 6: Manyara - Arusha The 7 days tours basically add a day on in Arusha which we don't want. We could stretch the budget slightly and do an extra day in the Serengeti but the tour operator that offers this doesn't have a lot of reviews. My question is: should we book based on just online reviews? Are there other things we should look for in a tour operator? Any help is appreciated.