well if your firmware is too high, I would go DSTWO. It is possible that they could make it compatible with newer firmware and you could take the chance on the gateway if your firmware is too new. Myself i have too much invested in the eshop on my 3dsxl, DSTWO seems to be pretty fast with updates, so if your one to always have newest firmware, you can most likely still use DSTWO but there always the possibility of nintendo finally locking the cart out for good.
once they make it so you can have multiple games on one sd card, i may consider getting another 3ds to play games not available on eshop, i like the ability to have multiple games on my system to be played at will (which is why i use eshop and dstwo), imo using gateway to play games with 1 on each card is no different than just swapping out with the real games.
but if your firmware is low enough and you wont miss eshop and dont mind using multiple sdcard or chaning the rom on the card than go for the gateway
