Different strokes. Trainer allows you to have very structured interval sessions, carefully calibrated, using HR monitor/power measuring device, which is harder to do on the road, in traffic, with varied weather conditions. You get sweat-soaked because you don't have the cooling effect and you're working at higher intensities - you need to use a large pillar mounted fan, at least 16".
On the other hand, if you have a decent hill, locally, and use that for interval training -ouch!
But for fitness and weight loss, you need a mix of both - using only the turbo might prove somewhat boring in the long run