If you are unsure between Drumit Three and Five MKII my advice is to buy the Five MKII from a big dealer with an easy return policy, as you are more likely to find issue with the 2box concept compared with a rival module (like the Yamaha DTX-PRO) than you are between the Three and Five MKII.
If you grow to love 2box concept - you'll regret not getting the Five MKII as you'll be hindered by 4GB once you start using SDSE.