Well, both normally have great temperaments, although womas do have a strong feeding response, womas would also normally get a bit smaller than Darwin carpets too, but besides all that, they are really such good beginner snakes and I highly recommend going with one, hope this helps
Only thing I can think that might help is height of your enclosure, woma's are more terrestrial (meaning hang out on ground all the time) where carpets are both terrestrial and arboreal so will climb branches etc and utilise the height of an enclosure much more than a woma.
Yeap you can get everything that you'd use for large enclosure and just put it in tub until big enough. You can even put tub IN the big enclosure when you've built it, saves on space. You'll be able to tell when little noodle is ready to go into bigger area, usually round year mark. Maybe earlier depending on personality and amount of hides etc available.