It depends on your play style. I've seen some people max out guardians for upgrades and then take the weaver skills to buff them every level to a nice effect. I've seen others concentrate on other traps instead.
Personally, I like to bottleneck orcs with a combination of barricades and tar traps, then take them out with wall blades and clockwork maces. So, I've purchased those upgrades.
I would stress that, in my experience, there are some traps that seem a bit underpowered and that could be ruled out when focusing on what play style works best for you and what upgrades to choose.
- The steam trap is something I've never used. It seems to lack sufficient utility.
- Decoys and boom barrels are things I usually only use when I have an extra slot available as a "get out of jail free" card.
- Because of how the spore mushroom works, it seems a bit too random to be reliable.
- The spring trap and push trap are very situational.