I think both are good boats. Personally, I'd prefer the Thunderjet if given those two choices. The engines are obviously the same, so it comes down to the hull and other components. I like the design/features of the Thunderjet hull more.

Sportjets are good for lighter boats and have a very good power to weight ratio. They are a two stroke so they inject oil They aren't as fuel efficient or as powerful as a V-8. They are very similar to an outboard except you have no way to trim the boat. Outboards have more fishable room and the four-stroke outboards are probably near the top in cost for power.

Mercury makes a stomp grate for sportjets. I think that would be a worthwhile investment no matter which boat your friend buys. It's possible that you'll get a lemon boat with a bad hull, but I think the chance is slim. The power for either boat is the same and is common. Most of the local dealers should be able to take care of any mechanical issue.