First off, in my opinion, you should have Hawk up almost all of the time in a 5-man or raid situation. There are exceptions, of course, which I'm sure someone will be happy to bring up. But, in general you should use Hawk to improve your dps, rather than Viper to conserve your mana. I use Viper all the time when soloing, but only rarely in instances/raids.
In 5-mans, fights are short enough that you shouldn't run out of mana. Drink between fights. Don't gimp your dps during a fight to conserve your mana.
In raids, fights can be longer, but you should expect to use consumables to keep from running out of mana. There are times when Viper can be used, but in most situations Hawk should be up to increase your dps output. If you're running out of mana in a fight, you're probably not using consumables, which you should be.
To address your specific question (my opinion, anyway):
Imp Hawk vs. Endurance Training - I'll pass on this one and let the BMs discuss this. Search previous threads, though, as Imp Hawk has been discussed before. Someone did the math on this regarding proc rate and just how much dps results.
Imp Hunter's Mark vs. Efficiency - In my opinion, Efficiency is a must if you go into second tier of MM; 5/5 for 10% reduction in mana cost adds up big-time in a long fight. I don't necessarily see this as an either/or question though. Imp Hunter's Mark is not really for your benefit, but for the party/raid's benefit. So, depending on your emphasis, you may want points there. Of course, a lot will depend on your build and where you're putting emphasis. In my case, I have 5 points in Efficiency and 4 points in Imp Hunter's Mark. I put those points in Imp Mark with the intention of helping the group and I try to make sure the raid leader puts me in a group with melee so the benefit is utilized.
Anyway, my 2 cents,
Azwing
Edited, Oct 9th 2007 1:17pm by azwing