We have heavy clay soil, so my experience may be different if you are in sand.  Here a 30,000lb dozer will take down most any tree. But for cutting and pushing a large volume of soil it's not heavy enough.  Building ponds and terraces is much easier with a 40,000lb machine.  I can drop the blade and just keep rolling soil with the 40k, while the 30k will spin out when the blade gets a full pile in front of it.  I would also recommend a dozer that can power turn.  Otherwise you lose 1/2 of your push power every time you steer.