Although, I’m just at act 4 NM, but why don’t you do both? I play a FO/CL-Sorceress (level 60 atm), for example. And at least in Act 4 NM just 5 points in Frostbolt (Synergy, while leveling to FO, so you don’t realy need that, if you’re high level enough. You can put those points in FO itself.) amd 11 in FO works pretty fine. I have 15 in CL and 10 in LM, so far. But I think atm to skill more a spell, which I also like, as Synergy for CL, instead of CL itslf. (Nova, in that case.) Although most players recommend the normal lightning over CL, I find the latter much better.
FO has this great advantage, that it only has one synergy, which even isn’t very much, compared to synergies of other spells, So if you realy want to max this, (which isn’t needed imho. at least until yet) you only need 40 points…
This leaves at least as much for the lightning-tree. And at least in Act 4 NM, this is very much enough, imho. BTW: I’m completely self-found. Using Ryme (ShaelEth)-shield with a spirit-sword as main outfit (and a Lore (OrtSol)-circlet and TalEth-Armor.)
Since this is an online-character, I’m refering to, it’s of course a “players 1”-game… If you play SP wiith players 8, it might be a bit less effectiv.
As I said, I’m not in Hell-difficulty, yet, But It seems, you don’t need to min-max… At least not in a solo-game.
Oh, and I’m using an Act 3-Ice-Merc, meanwhile (after using and act II-prayer-Merc) And this functions, too. (But don’t use the fire. Act 3 Fire is useless imho. Even lightning is better, But ice is the best.) But as soon, as I find a 4-socket Polearm, I will switch to act II holy-freeze, Although Prayer is realy comfortable imho. But with Insight, this isn’t necessary.)