What would a begginer have a harder time facing?
More theory d4 or e4? More ambitious?

There's not a lot to say about a position only 1 move into the game. People sometimes say e4 is tactical while d4 is positional but that's a very rough characterization. All chess positions involve tactics and all positions involve positional aspects. All openings (taken from move 1) have more theory than you'll ever remember.
It's a matter of preference. Try them both out for a few games and see which gives you positions where you're more comfortable. That's one thing to ask yourself... if the middlegames are comfortable for you to play. Another is if your results are good out of those positions (the answers to these two aren't always the same).

Most chess instructors recommend e4 for beginner and intermediate players. When your performance is not doing well, maybe it's time to switch to d4 and see if results improve.

Just play and see what you like best. I don't think there is a rule of thumb to follow here. As for the amount of theory, I don't think you should bother to much about that for the moment. Just follow the simple developping "rules" and you will reach a playable position.
I would say e4 has more theory. While people may argue that the average d4 line has as much theory as the average e4 line, they aren't taking into account the sheer MASS of e4 lines. Typical d4 just includes queen's gambit accepted and decline, and maybe a KID or nimzo indian. Perhaps a benoni or benko gambit, rarely.
On the other hand, e4 has tons of lines. Assuming you have your standard pet response to e5 and N6 which is probably as complicated as any of the previous d4 lines, you also have to contend the many Sicillian variations (the sicillian itself probably has more theory than all of the d4 lines), as well as the Caro-Kann, the French, the Petroff, the Pirc, and the Scandinavian, to name a few.

There's plenty of theory on both lines. Both lines can be played very aggressively. Generally, 1.d4 defers the main battle while keeping the center somewhat closed while 1.e4 is more direct.
The openness of 1.e4 is often misinterpreted as being more aggressive. But when top GMs absolutely need to win, they tend to play 1.d4 more often than 1.e4. For proof of this, see the last few world championship matches.
How do you decide between e4 or d4??? how is it possible. Which has more theory? which trys for a win harder? How do you decide? which is more ambitous? what questions should i ask myself before picking?