Hmm it's been a while since I've been leveling a summoner and it has changed so much I'm not entirely sure what's the best way to level up. I think it also depends a lot on the playstyle.
The safest way to lvl up perhaps would be to just roll with Hellion and take the rubies that remove cooldown on Vampirism/Life tap and use phlebotomy for determination and the occasional Howl of death/heal but this is very single target and to me it seems terribly boring
Lurker seems better if you just want to pull multiple mobs, dot/volatile infection/blood ties everything and heal yourself like crazy (since Hellion wouldn't aggro that all anyway) and if you take the rubies that remove vampirism cd here as well the single target dmg will be ok, just prepare for the fact you'll have to use heals a lot and/or renew blood aegis. If you do take Lurker then yeah I'd say take Hellion rank 1-2 since I imagine leveling without any pet up to lvl 28 - or around that - could be pain.
I think on higher levels Hellion's aggro is pretty low anyway but I have never tried the Hellion aggro rubies.
Diament's guide includes a leveling build of the second option here, you could follow it