I think it really depends what you want to get out of the bed, how much you can spend at the time & what you think your
DS will be able to handle.
Our
DS only went in a bed just after 2 & he's a big boy so a toddler bed was going to be no good for him. He has his own savings account that we regularly put $$$ in so we thought why not use some of that to buy a decent sized bed that will last. Then we don't have to go down the "what bed path"again for some time. We got a white timeber Kind Single & I love it!!! He doesn't fall out as we have a rail, gets in & out no worries at all. No probs getting sheets.....Target has an awesome selection in sooooooo many colours going from single, king single & so on!!! Also it's great as we fit in it too if we need to cuddle.
As for bunks, had we known we were having another boy at the time of buying we prob would have gotten bunks that can be pulled apart as they will eventually share a room. So that's something to think about.
I wouldn't spend too much $$$$ on a themed bed that my later not be as liked by either you or your son IYKWIM.