I have the lvl 24 pet adept 3 and love it. I would think about getting the lvl 25 pet buff if I get my hands on another palladium or jasper.
in my experience the jump from app3 pet to adept 1 is not much, but from app3 to adept3 it rather noticeable, and since i have NEVER seen adept1 24 pet, i'd say adept3 24 pet is rather important. pet buffs and heal are important if you solo, nukes an dots are important if you group, since accumulatively(*sp English was never my strong point math is) dots and nukes will outdamage your pet, having better nukes and dots is more important while grouping then having your pet buffd or healing it. (not to say pet buff is not important) but while solo since we have to watch our DPS anyway, the pet buff and heal means more. also remember to buff your pet b4 you send it in since buffing it while fighting will just give you that much more agro. hope this helps you make your decission.
The lvl 24 adept 3 pet hits for between 30-50 dmg per hit, which is very nice. I recently upgraded my favor of the fallen from app2 to adept1 and saw a 13 point increase in power over the app2, and also a 31 point increase in the resists (think mental and disease, but can't remember). When I hit lvl 25, I will definately be looking to upgrade the buff, since my pet is dealing quite a bit of damage now, I'm sure he'll be even better with a better buff which stacks.
I'd recommend upgrading your lvl 24 pet for sure, i did him and hes now to the point where he can hold aggro fairly effectively, and I can nuke dot almost chain-style with Boon Adept 1 up... and he hardly loses health to yellow cons. The other one i'm going to recommend upgrading is your level 29 nuke - Pestilent Blast. It hits decent at adept 1, but bumping it up to adept 3 really seems to help out.
Oh hmmm.. what is steel cluster used for then? I bought that thinking it would be used for the adept III pet upgrade