Ok, so I finally gave this a go. Here are my findings.
Trained 4 dwellers to max. stats at level 1. Meet our 4 heroes:
All are equipped with a Heavy Wasteland Gear (to get max HP gain per Level-up).
- Mr. Worker: He will be working in a Nuka Cola Plant with 5 other Dwellers
- Mr. Wasteland: His task is to roam the wastelands
- Mr. Soloworker: He well be working in a Nuka-Cola Bottler on its own
- Mr. Outfitter: He well be working in an outfit plant all by himself, building a T-60F Power Armor.
Here are the stats after ~1 hour (Levels: 3, 2, 3, 4):
After ~10 hours (Levels: 13, 7, 13, 6):
After ~15 hours (Levels: 17, 8, 19, 7):
As you can see, the experience gain in crafting rooms seems to be at least double the amount you get from normal production rooms.
The levels of the dwellers that remained in the vault could potentially be even higher, because they stop gaining experience once they level up (you'll have to click them to continue). And I'm sure I wasted some precious time by not opening the game at exactly the right moments. I guess this also explains the discrepancy between the two dwellers in production rooms…
Conclusion: Experience gain is really good in crafting rooms. So best stick dwellers in there that aren't at level 50 yet. It's no alternative to leveling in the wasteland though, because you'll have to babysit the dwellers (clicking them to level up) and the experience gain doesn't ramp up like the experience gained in the wasteland.