Valenar Elves would be a lot less nice, but since they have some serious ethnic hate for the hobgoblins, they might be more likely to fight for the PCs. Conversely, the Red Hand is likely to hit them first, since the goblinoids really hate the Valenar elves for carving a piece out of their empire.
Other issues are as follows:
1. Valenar elves prefer open steppes. Why are they in a swamp cut off from the plains by a large, haunted forest?
2. Why there are Valenar elves there in the first place? This is highly dependent on where you actually place the Elsir Vale. It wasn't feasible in my campaign since I set it in the eastern tip of Breland (Dennovar was replaced by Vathirond, the last city before the Mournland). It was on the other side of the Mournland from Valenar.
3. While the Valenar elves hate hobgoblins, they don't have good relations with many other nations. They regularly raid Karrnath, for one. If you set the Vale in other possible settings (out-of-the-way places like eastern Breland or the Eldeen Reacheers), you'll have to explain why there are humans there, or change the inhabitants of Brindol and the other towns.
4. The owls only matter if the PCs get them, since they have to really impress the elves to get them. Same thing with the Valenar horses, as those are custom bred and not so lightly given to outsiders.
Valenar horses, strictly speaking, are faster (covering 64 miles in a day while the owls would cover only 56 (7 mph x 8 hours). However, they can avoid most encounters by flying, and as mentioned, can travel in a straight line. That saves a lot of time.