It really depends on what its being used for if resin or plastic is better.
Resin is good for detail parts (cockpits, landing gear wells..) but I'm not fond of full resin kits, certain components in resin, especially large thin ones like wings can be prone to sagging over time. With resin there is also the possibility of large air bubbles lurking under the surface of a seemingly solid part.
Because a lot of resin kits come from the cottage industry and garage companies, they can be a hit and miss affair. Do your research before commiting money to resin products.
As for plastic, I'd say its getting better all the time, mold making equipment is getting more exacting, and plastics are being better developed to flow in the molds better during the molding process to better capture the finer detail that is possible with computer driven machining equipment making the molds.
I prefer to limit my use of resins to cockpit and landing gear areas, and keep the majority of the finished product in plastic.