I would say Cresselia because you need a Special wall for your team since Forretress can tank enough physical hits. Although Umbreon and Cresselia share the same base special defense, Cresselia has a better bulk overall. Umbreon is still a great Pokemon to use since it has acces to both status recovering and HP recovering moves+ acces to synchronize meaning that trying to cripple it practically cripples you whiles it can heal itself with moonlight and heal bell. It doesn't only recover itself, but also it's teammates making him a great Pokemon to use but since your are using OU I recommend Cresselia. However Umbreon can do a good job in OU when used well but it has too many Pokemon that can check it making Cresselia a better choice.
If you ever want to try an Umbreon set, then I would recommend the Physically Defensive set which would be
Umbreon @ Leftovers
EV spread: 252 HP / 252 Def / 4 SpDef
- Heal bell
- Foul Play
This set works really well and can help you keep your teammates alive and tbh, Umbreon would be good for your team. It has a great Special Defense so even physically defensive, it can tank hits really great. And your sweepers can't afford to stay crippled
So for your teams well being I say Umbreon. Chansey can do the trick too but you asked for Cresselia or Umbreon so I will stick to your request.
For any other team I would recommend Cresselua because a 120/120/130 base bulk is pretty impressive. It is hard to keep Cresselia once crippled but Doctor Flame's recycle set can help you keep it around for very very long
Cresselia @ Lum Berry
Calm Nature (for your request's sake)
EV Spread: 252 HP / 56 SpDef / 200 Def
- Toxic/ Hidden Power Fire
This set can outstall anything but can be checked by Scizor when you aren't running Hidden Power Fire, but you can always switch into your trusty Forretress.
A great set, but it doesn't support your teammates like Umbreon but for a Wall, this thing is pretty much it.
Like all walls you must be wary of taunt since it could cripple you. But just to not make you a taunt bait, psychic and hidden Power grant you enough coverage :)
- If you want to support your sweepers, go for Umbreon
- If you want one solid wall, go for Cresselia :)