Everything is different, and I would recommend none of them to a class player.

Their main difference is that theory ends around move 30 for 6.Be3, while for 6.Bg5 it ends around move 40.

A class player like you should not care at all about theoretical lines going over move six. So...

It's not so difficult to explain the differences, but it's much easier to say to you that a non-pro player should not care about such very complex and over-analysed lines.

A 1900 player may indeed start wondering about the differences and fine details behind Bg5 and Be3. But I'm talking about 1900 FIDE- neither USCF, nor chess.com

I don't think that the difference is that great, around 110-130 points would be  more accurate. Anyway, regardless of the actual player strength, if you want to learn these two lines really well, you must dedicate a lot of time- and I mean A LOT. Better play something less critical, but easier/lazier to master.

No, but they should stick to something simple to master. True, almost all the reliable Sicilian variations have a respectable amount of theory, but they aren't all "the same".

Yes, not so much theory, and while the mainlines are "equal" there is quite some play left for both sides.

I might have the chance to intoduce you to an IM who hardly knows any opening theory. Sometimes he got crushed in a few moves because he ignored existing traps- yet he had reached at a time a rating around 2440 by playing stuff like 1.e4 e5 2.Nf3 Qe7 and such.

May I kindly ask what YOUR rating is and how much studying opening theory had benefited you? Take your time...

For the record, you remind me of one of my best OTB "clients". He is always booked/armed with the latest opening trend and GM novelty. He has scored against me half a point in nine games, and the recipe was always the same: I played complete nonsense in the opening, borderline not losing by force, yet complex and non-book. Most of the games ended in less than 25 moves. He is a FIDE titled player, yet he plays badly- it's just that his opponents do not apply the correct recipe.

Do you really envy such players?

You can find some semi-correct sideline to play... but entering stuff like the Poisoned Pawn, where endless sheets of mixed human and computer generated crap is widely available, sometimes exceeding move 50, makes no sense at all for a class player. Same goes for mainline Dragon, as well as other openings, e.g. the Ruy Marshall. Even if you manage to absorb all that crap, it's not nice polaying against a woodpusher who just happens to eat books for breakfast, and draw or lose against him, while you could have a very easy life against him using 1.e4 h6.

I would suggest (for Black) to class players either the Kan or the Sveshnikov- the former is mainly a matter of sane positional play, while the latter is quite theory-heavy but easy to understand, and it will also help you improve your positional understanding (at least it will teach you that offering your opponent d5 on a plate, or leaving the d6 pawn permanently weak aren't the end of the world, if they are dynamically compensated).