Personally, I'd pick the Russian/Chinese branch (Gvosdika, Akatsiya, PLZ-89) even though it requires more XP to grind through.
- Gvosdika is fairly accurate and has good RoF, making it relatively easy to use
- Akatsiya gets a 3-round autoloader, allowing it to dish out lots of damage in a short period of time (and be somewhat effective when doing direct-fire)
- PLZ-89 gets a 3-round autoloader with better accuracy, RoF, and aim time. Probably one of the strongest SPGs in the game tier-for-tier
If you can go down both lines at the same time, you could also go down the M109/Palmraia line. The M109 isn't particularly good, but the Palmaria is pretty decent.
Once you get to tier 8, your only choice for a SPG is the flaming dumpster fire that is the Paladin, so it might be worth considering going down some of the other non-SPG lines to unlock your first tier 9 SPG.
For tier 9, you effectively have 3 SPGs that are all very good:
- PLZ-05 with a 4 shot autoloader, has amazing burst damage but terrible accuracy and aim time
- PLZ-05 with a single-shot gun, has good RoF and great accuracy and aim time
- PzH 2000 with a 3-round autoloader, has decent burst damage with good accuracy and aim time
Tier 10 has the Cent 155 with good RoF and little else. It's not terrible but you're probably better off using a tier 9 SPG if you want to get anything done.