I like to have one of each on my team, but if I absolutely have to pick between the two, Id go engineer, at least until the Recoiless rifle upgrade. I like engineers because I know exactly where the grenade will go, and, if sapper is picked, what terrain will most likely be damaged.
Rocketeers are fun however for those “fuck it” moments, when you need to deal a lot of damage to a large number of nearby enemies. The wide explosion radius also helps a lot with the native tile spread they have.
Which to you prefer?
Depends a bit on alien tech level, enemy density and the map itself. In long war rebalance you get a preview of which map you’ll play on before the mission starts.
Very early on 1-2 rocketeers can guarantee a victory thanks to the power of the rockets. They fall off in the mid game though. On swarming abductions I’d always bring a rocketeer because you’re bound to hit something and because you’re often under high pressure to immediately clear the area next to the landing zone before additional roaming pods patrol into you.