i think red pike uses a little more balanced build, and i have already seen pure str pikers doing a really good job
Red Pike was 21/18 with full IF, PS and ATH. 3 WM.
I've only used 18/21 and 21/18 myself and loved them both (full IF, PS and WM). Remember that the insane length of the pike will slow you down a lot, so even if you stack agility and athletics, it doesn't mean you'll outrun most medium infantry, however it gives you more to work with in 1v1 and you're able to make more attacks overall, and cover more ground when harassing or dealing with cavalry. With more strength the survivability will be higher, of course. The damage depends on how good you're at abusing the speed bonus. Strength is nice if your timing is not the best, but the better you are, the more you can get out of the extra agility. WM stacking might be useless in other melee classes but it helps quite a bit with pikes, imo. Should go for it in your build.
I like a balanced build, but try out some with your STF before making your final decision for your main.