Either is fine.
I have a raised floor where I'm installing a toilet and coming right out of the toilet i can do one of those longer 90 elbows or (2) 45's for a gentler angle coming right out of the toilet mount. The advantage to the 90 piece is that i can get a steeper slope by an inch or so.
Is there really much difference between the 2 ?
Under normal circumstances will the 90 possibly clog in time ?
WHY do you "need" a steeper slope with the elbow? If you have the "proper" slope it doesn't make any difference which fittings you use.
Licensed residential and commercial plumber