Me and my friend are in the same boat as you, and the majority of the answers have already been provided for you. I will give you a few of our situational decisions, and maybe it can lead you to a properly based decision.
Class Size: This is a huge thing for us, we have been racing everything together all of our lives, and we love competition. ALSO, we just recently got done building 2 SCCA STL class honda civics, and the class in florida literally died over the off season. My thing is I will ruin 10K worth of racing machine if I am out there with 15-20 people, but I refuse to risk my money to race 3 people.
Availibility and knowledge: Where we race in Ocala, FL. Tony Kart is the go to chassis. Allbeit Tony Kart, FA Kart, Cosmic, Expirit, but all tony kart nonetheless. Also our home track OGP only stocks TK parts because of this. Their travel team, and race team all run Tony Karts, so clearly this decision was pre-determined for us. I refuse to have a race weekend ruined, or ended because I broke a part I cannot get because nobody carries it. If I break something and they are out of stock, well that is just dumb luck, and I chaulk it up as a lesson learned. Obviously also because they race, own, operate, and sell TK clearly the knowledge to help us with the tuning learning curve is there.
Reliability: I have read great things about all the TaG, and Rotax FR125 motors. I have also read bad things. The bad things usually seem to stem back to whom the builder was, more so than the product itself. I believe if you find a QUALITY engine builder, its just like a real car engine, or dirtbike engine. Everything is only as reliable as the maintenance program you put fourth.
For us we chose Rotax Masters class, we are both 30 but we meet the minimum weight of the driver without helmet of 190+. Another reason we chose the Rotax motor is that it essentially gives us more classes to run. I can run TaG Heavy and Rotax Masters during the FKCS, if I wanted to. All without changing a thing, will I be as competitive? Probably not, this will be my first year in a kart. Closest thing I have is 1/4 midgets, see the similarity at all?
All-in-All it comes down with like everything you do. What is popular in your area, what type of support can you get, and what is the best bang for the buck.