Always EXP, imo.

I usually go Exp.
Zilean is the only other thing that can give you that bonus.

I usually run with enough mana regen runes and masteries that I don't mind not having an extra 100 mana.

If I am solo, the Exp gain is very helpful.

The 100 mana doesn't give me enough to harass the enemy enough to make them lose the 5% Exp

But at level 18, the Exp becomes completely useless.

Its really all up to you.

I've honestly went 9/0/21 for Heimer becaues I want to rely on flash to be up to get out of bad circumstances, and the cooldowns are nice.

As well as it helps his early game of manahogging.

That way I can take something OTHER than clarity, aka flash teleport.

Having +EXP for Heimer is pretty important since he's such a powerful farmer and lane pusher he should be able to get a lot of uninterupted experience gain while the rest of your team is sorting team fights, ganks, and such early on.

Having +EXP is really good in general since it helps widen level gaps between the teams and is entirely essential for junglers to reach level breakpoints based on number of neutrals killed.

+100 more mana at the very very end of the game is pretty inconsequential, especially if you already have zhonyas and 2 AAs.

To be fair, getting to level 6 doesn't really do much for Heim. Sure, the CDR is nice, but until they fix the targetting on his turrets the active is only good in limited situations.

However, you still want the 5% exp, because the faster you get to level 9 and red turrets the better your farm is going to be.

I would say exp, as you lvl faster, so you have better odds of killing enemies sooner, and feeding faster.
100 mana is ultimately irrelevent as its barely 1 spells cost. and 7 AP can be bought with 100 gold (ratio) so exp is prolly better. If you couple that with a zilian on the team and some runes, you can hit lvl 14 before your opponent hits 10.