I cut my teeth in BBQ on a Brinkman Cimmaron offset. It's 1/4" steel, welded firbox. I have had it since the late 90's. Back then they were about $800 - $900 new. My dad bought it for me for Christmas. Even with the heavy steel and welded construction, it took some modifications to get it to work well. I cooked some kicka$$ BBQ on that pit and really learned about fire control, thin blue, and a host of other lessons.
Now take a look at my signature. I own 2 offsets (1 I bought because it needed to be saved from a slow death due to neglect) and I own 10 WSMs.
Does that tell ya something? (Other than I'm a WSM hoarder)
Can you cook great Q on a small, inexpensive offset? Yes, if you don't mind tending to the pit every 30 - 60 minutes.
Can you cook great Q on a WSM out of the box? Yes, and when you have learned the WSM you can tend the fire every 12 - 16 hours.
There is something special about cooking on an offset, and I commend you for thinking about getting one. But it all comes down to one thing: VALUE. When you spend your $300 on a pit, what's the best value? For you, the desire to cook on an offset may outweigh the need for long cook times. Therefore, the offset would be a better value for you. If long, unattended cooks are your goal, then spending your $300 on a WSM would be the better value.
Did I make sense?
Anyway, us Weber guys have a saying: "Buy the best, and only cry once."
BTW, at my first comp, cooking on the oldest WSM in the world, I placed 6th in pork and 5th in brisket. Since then I have placed second in pork and first in brisket, all cooking on WSMs.
My Boys: Joseph (EI); Andrew (EE); Daniel (EE)
My Toys: 10 WSMs (B, C, DH, DA, EZ, DZ, EO, DU, DR, EE/DU mutt); Brinkman Cimmaron; Chuckwagon Cooker; 4 22.5" Bar-B-Q-Kettles; 1 18.5" Bar-B-Q-Kettle; 1 Red OTS (H); 3 WGAs (EZ, DE, N); 3 SJPs (DT, DR, DR); 2 Smokey Joes (A, K); Genesis Silver C (DU).
"For when democracy becomes tyranny, those of us with rifles still get to vote"