Done one of each, nobody outside of triathlon knows the difference (anything in that distance gets called an ironman), so pick the race that matches your ambition or skills best, and that has a good reputation.
The little touches on Mdot are good (name on your number so people who don’t know you can’t shout support), but you pay a price for that. Ironman races are the only way to get to race in Hawaii (world champs) if you’re that way inclined.
I’m going back to half IM distance next year, and will be doing local, non Mdot branded races as supporting local organisers is now my priority.
If you want a recommendation, look at Outlaw for a fantastic IM distance UK race.