Hardware about the slot 2 cards (especially ez3in1)

[AsianMemories]

where are the save files saved? on the slot 2 cart or the microsd in the slot 1 cart? im leaning towards the 3in1, but what do you guys think of the battery saves? is it a big problem? how can i compensate for this? im not sure but i think theres a backup tool

and if i want web browser+roms then 3in1 is the best choice right? sorry for all the questions. thanks.

also, is there a builtin action replay in the 3in1? if not, can i flash a game to the cart, then use it with a physical action replay device?