The steel shield will slow you down... a lot, so even with very high athl you will still have a hardish time getting great speed bonuses, so I would suggest as another option 12/24 or 12/27 (intermediate ones to the ones you chose):
Level 30 (4 420 577 xp)
Strength: 12
Agility: 24
Skills to attributes: 2
Ironflesh: 1
Power Strike: 4
Shield: 8
Athletics: 8
Weapon Master: 8
One Handed: 173
Strength: 12
Agility: 27
Skills to attributes: 8
Power Strike: 4
Shield: 9
Athletics: 9
Weapon Master: 1
One Handed: 119
This allows you to go the kinngrimm/leoking way with steel pick, niuweidao, italian sword, elite scimitar, winged mace and some other very good weapons. I have used this build a few gens (with bucklers and heavy round shield, never used steel shield) and it is quite fun
And btw, 15/21 is in no way STR whore.
Regarding what class of both, just play with both for a while and decide which one you prefer. I for one cannot play crossbowman, but it is up to preference. Can you play being hidden most of the time and going out, shooting and then back into cover? Or do you prefer to be in the front line of the battle, tanking hits for your teammates, protecting them from ranged and smacking their faces afterwards?
For siege, steel shield comes in handy to stop those pesky maulers, while with an arbalest you choose your safe area and enjoy seeing people dying to your headshots.