So here's what I found about exp grinding:
228 hard gives approx 10.5k exp
239 easy gives approx 11k exp
239 medium gives approx 16.5k exp
239 hard gives approx 22k exp
With these values in mind, and my personal times (before I had RT1 gear):
228 hard takes 3-4 minutes on average
239 easy idk, haven't played enough
239 medium takes approx 7-8 minutes on average
239 hard takes approx 10 minutes on average
This means that 228 hard yields approx 2.5 - 3.5k exp/minute
239 medium yields approx 2 - 2.4k exp/minute
239 hard yields approx 2.2k exp/minute
(I used ram for 239 and TA patch for 228)
With this information, I'd say without doubt that unless you have a good team that can complete 239 medium in less than 5 minutes, stick to soloing (or better yet, using a team on) lv 228 hard. With a good team on 228 you can hit upwards of 4.5k exp/minute!!
EDIT: Ok, so RT1 changes my times considerably.
Now I'm getting 2.5 - 3. 5 minutes on 228 hard. Here are my new findings....
IF you get similar to my times on 228 as your average (we'll say 3 minutes to make things simple), then here's te breakdown.
Don't bother with 239 easy unless you can get 3 minutes or under
Don't bother with 238 medium unless you can get 4.75 minutes or under
Don't bother with 239 hard unless you can get 6.25 minutes or under
This info only applies when you are soloing. Times vary greatly when using teams, due to differing skill levels.
Also, if you're capping exp, like on a weekend, try and do the level that lets you reach near cap in the least amount of time (this goes without saying, but I thought if mention it...)
Anyways, thought you guys would like to know. I might make a guide that includes several more levels when I have time.
Ps: I have t3, guild buff, and member to get these rates. If you don't have member, divide by 2.
EDIT: can someone move this to Tips/Guides? Didn't mean to post it here...