Imo, playing a strength based shielder needs IF.
It's a tad bit different now (with the lighter armour), but do you really want to sacrifice 6! skillpoints for 1 measly point in powerstrike?
The best options I can think of:
Agility build: 15/21 (or maybe even 12/24 if you're using a steel pick or warhammer. I tried it and you don't glance on plate with a steel pick and 4 PS. You just need a lot of hits to kill the dude. I'd rather not convert skillpoints with this build, as you need just about all of em. Better wait for level 31.)
Strength build: 21/15 (Like the build mentioned earlier, but with 7 IF. Since the strengthskills only require 2 skillpoints instead of the 3 for agility, it's no problem to convert 2 skillpoints into your last strength point.)
Balanced build: 18/18 (Again, I'd rather wait for level 31 to finish this build than convert skillpoints. You need em all.)
In all my builds I max out IF, PS, Ath, Shield and WM. Personally I think IF is worth the investment, but you might disagree on that.
The biggest advantage of a strength build in battles and during sieges is the little amount of hits required to kill opponents. You can run up and down a wall while slashing at stuff around you and most will drop dead in 1 or 2 hits (NCS/Steel Pick/Broad 1h battle axe). As an agility build you're much deadlier in 1v1 fights, but the problem is that you don't always have time for that and fights often get interrupted.
It's cool if you can hit 4-6 guys before going down, but if you didn't kill anyone it's a wasted effort.