The Grille is the best Tier 4 arty IMO, but unless you've already progressed through the Wespe, you're going to have to go through it first. The StPzII is basically just an upgraded Bison as it still has the same gun.
I just started playing the Wespe this week, and I like it, although the aim time seems long compared to the Priest (since they both shoot 105mm shells they have similar damage), so I'm going to buy a GLD when I get the credits (HAD to buy a rammer for my T29 recently).
I'm going Hummel via Grille also (no way I'm doing all that XP with the Pz3/4). Plus I want the Grille.
The StPzII is difficult since beginning at Tier 3, arty is expected to be able to hit most of the map, and you still will have the same half court range with StPzII that you did with the Bison. Unlike the M37, this never improves as there's no "second gun" to research.
I haven't played Russian arty and have no interest in going down that line at the moment, so I can't speak for them.
Edited by Pavol_Preznik, Jun 02 2012 - 21:04.