That will be up to you man. If you want the shorter batteries for when your out of the house, so you won't look so wonky walking around in front of people. the 18490 mode will basically fit in your palm so it'll almost be completely concealed. The 18650 mode is a little bit bigger and it'll be obvious your vaping from a device yknow? I find that in a vv mod, an 18650 basically lasts me all day. The 18350 batteries in my mechanical mods last me around 3 hours before the voltage drops off pretty hard. So the 18490's will probably last about half a day I'd guess? When using SR cartos at a reasonable voltage.. So thats up to you. My Mini takes 18350 batteries and I'm guessing they will last around 4 to 5 hours each since the voltage drop of the batteries in my provari will not be noticeable until right before the battery dies. (unlike a mechanical mod) So if I used my Provari all day into the night, I might have to change the batteries a total of 3 times. (but I wont, because I'll use my other mods)
So to sum things up, my guess is that the 18650 batteries will last you all day if your a moderate/sometimes heavy vaper like I am. The 18490 batteries will last half a day, so you'd switch them at least once throughout the day. And, like I said, the Provari is only a teeeny bit shorter with the 18490 batteries, and it doesn't really seem all that significant of a length change, but, it can make a difference if your the type to want a mod to be as small as possible when out on the town.
If you want, I can suggest you look up on youtube the two modes. The device will look larger than it is on your screen though, so you really have to look at the reviewer's hands and then compare them to your own for scale. By watching reviews of it in the 2 different modes, you'll be able to see if its worth it to use the smaller batteries or not. I don't really mind carrying an extra battery with me while I'm out, so I got the mini and I'll probably use it in mini mode without the extension cap because I have other mods as well.
Heres a great video you may want to watch before or after getting your provari to see the size comparisons:
https://www.youtube.com/watch?v=FG9DQbowrDA&feature=related