...would you say is ultimately more profitable?

This coming from a dunmer thief, so keep that in mind.

The quest rewards are the same (except one minor Imperial glitch, but it's nothing you'll miss). The only difference is how certain people treat you - and usually that doesn't manifest until there is a clear winner in the war (end of the questline).

Dunmers would generally be against Ulfric (talk to the Dunmer in Windhelm and you'll see why), but my Dunmer warrior joined the Stormcloaks anyways and was warmly received and treated as an equal. Both sides dislike thievery (of course), but
In the end, Imperial armor sells for more than Stormcloak armor, so I guess the more profitable would be Stormcloaks.

Neither. Money is easy to come by. On one of my characters i completely ignored the main quest and the civil war questline and i dumped that character around level 30 with three fully furnished houses and over 500,000 gold in cash.

And i didn't cheat or grind skills ...

lol, you clearly picked up every item you could though.
After 100hrs or so and being level 47 my character has about 30,000.

Yes, i'm a bit neurotic.

That character was a hunter/adventurer and i sold everything i could schlepp. Made leather armors out of hides, made jewelery out of ore and made potions.

It's very easy to make lots of money off loot. By the time you walk from one city to another you have enough stuff for several thousand septims. Then do the same on your way back.

From an in-character perspective, politics are not your battle to fight. Plus, with the war comes business opportunities. And you do like business opportunities, don't you? If you didn't you'd just have stayed in Almalexia.

Yes money is easy to come by, but I mean more from the characters perspective...

Within the game or in an RP sense?

More of an RP sense.

Empire. No contest.

Currently Skyrim's economy relies heavily on trade with the Empire. A Stormcloak victory will likely be devastating to Skyrim's economy as it means severing those ties. And it would likely take Skyrim years to recover.

Conversely, an Imperial victory keeps the trade coming in, and with it the wealth. More wealth means more to pilfer and pillage.

from a rp point of view, id say the empire.

Reason is, stormcloaks win they got tons of gold from their silver mines alone, but under an ulfric empire, things would be strickter.
Whereas the empire is has the history of officers being corrupt on many levels, plus I see it being easier to be profitable with less risks umder the empire.

Stick with the Thalmor.

Let the stormcloaks and imperials kill each other. Loot their fallen bodies and make a ton of money.

Apart from the small fact that my people and your people hate each other, the Thalmor view the Dunmer as vastly inferior, almost as bad as the beast folk.

The dunmer have forsaken the beautiful merrish ways of the aldmer! Suffer not a Velothian heretic to live! So proclaims the Great Thalmor!

Stormcloaks. Tullius was gonna chop your head off then makes a snide remark about it. Get hjerim in half the time also.

A Dunmer would probably side with the Imperials.

This. Pull a Thénardier and just let them sort their affairs out themselves. As long as the war's going on, the guards are distracted, resources are scarcer, demand is higher, and the prices are elevated.

In the end your thief will want the Empire to win out, but not until you're ready to join the highest ranks of society with your pilfered gains.

Do you like blue, or red?

Stormcloaks are more enjoyable than Imperials but legion pays you a little more in gold. On both sides you have potential loot source: most of Stormcloaks have a scaled helm in their inventory and some Legionaries have an amulet of random divine. This loot gathering favors better Imperials because you always will found more scaled helmets on Imperial side than amulets on Stormcloak side.

Once the Thamor find out you are a Shezzarine heretic, they will just have the Empire sieze your assets and let the corrupt Imperial Justice System do what it does best, this time you will definitely be on the list.