Although countless arguments can be made in favor of not charging the batteries inside the mod, IMO it should nevertheless be noted that the Vaporesso GEN X mod does in fact do balanced charging, correctly, despite that there is still a small imbalance there.
As for which batteries to choose, the most important factor is how many watts you vape at. As a general rule of thumb, a pair of Sony/Murata VTC6 or pair of Samsung 30Q batteries in a regulated mod can handle up to 90W with adequate safety. But the vaping time you will get is part dependent of the efficiency of the batteries, which, in turn, mainly depends on how hard you push the batteries. If, for example, you vape at 80W on a pair of Samsung 30Q batteries with a regulated mod, then you should be able to notice a bit of improvement by switching to a pair of Molicel P26A batteries─in spite of the fact that the latter choice of battery is only 2600mAh vs 3000mAh for the former.
That's just because the latter is a 25A battery vs only 15A for the former, so a pair of the latter ones can handle up to 150W (i.e., 25A multiplied by three, multiplied by two batteries in the GEN X) with adequate safety in a regulated mod, which also explains why vaping at 80W will be pushing them less hard when compared to vaping the same mod at 80W on a pair of the former ones, as pulling 80W from a pair of batteries that can handle 90W (or 15A times three times two batteries) is pushing the batteries much harder than is pulling 80W from a pair that can do 150W. The efficiency of a battery improves if you push it less hard. So much so, a 2600mAh battery can run for longer than a 3000mAh one.
Further, if you don't give the batteries much time to cool back down again each time after you take a few hits, then a battery with a noticeably lower amp rating is going to run at higher temperatures as a result from their being pushed harder. The warmer the battery gets, the faster it ages. As long as the batteries don't reach above 45 degrees Celsius, you shouldn't expect the batteries to age faster than what could be considered to be "average level" aging speed. But you can still get them to age much slower than this "average level", i.e. either by choosing to wait longer between hits or by choosing to buy the batteries that have the higher amp rating. Pulling less amps from a battery also translates to pushing the battery less hard. So, even though keeping the battery temperature down reasonably low above room temperature level is still the most important factor when it comes to battery aging, staying well below the amp rating of this same battery also plays a part in this regard.
Finally, if you don't charge the batteries all the way up to 4.2V each time when you charge them, like for example if, usually, you only charge them up to 90%-95%, then they will age even slower. And, if you also try to avoid charging your batteries far too long before (e.g., more than a day before) you will actually be using them, vaping at 80W on the GEN X with a pair of Molicel P26A batteries should make these batteries last for several many years to come. It could be easily up to 4-5 years if you take excellent care of your batteries.