Think about if you want a low DEX build - ranger combat feats don´t need prequisites, so unlike a fighter they can go TWF without even having a DEX modifier. If you want that, you choose the regular two weapon fighting style since all feats on the shield style combat list are without DEX prequisite. (however, rangers get Evasion, so a decent DEX is certainly no loss)
TWF style can net you more attacks at a low Dex modifier because ITWF and GTWF are style feats and style feats can ignore pre-requisites.
That being said, I think ITWF and definitely GTWF can be over-valued in terms of the true impact of additional attacks.
As such I think the benefits of Weapon & Shield are probably greater because you can get Improved Shield Bash, Shield Slam and Shield Master with bonus feats allowing you to use your standard feats on critical feats like Iron Will and Power Attack.
Just remember that you need to stay with a light shield because the heavy shield penalties are horrific. Even with shield master they impact the primary weapon.
Go with two weapon fighting. Why? Because it does both.
At low levels, you can decide to:
a) use shield as shield, giving you hand weapon and shield
b) use shield as weapon, giving you TWF.
This is a LOT of versatility, and it only gets better as you stack up on shield fighting feats.
Remember, you also have c) drop shield, take out greatsword. As a guy with good BAB and strength, you'll do just fine.
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My understanding of the heavy shield penalties is that it really just forces you to a light weapon in the 'other' hand. Light shield and longsword or heavy shield and shortsword.
Well, interestinlgy enough with the Ranger Shield Style able to ignore Pre-Reqs, you don`t need to 2WF at all while using the full range of Shield Feats. If you want, your Shield Bash can be your primary attack, and not use 2WF at all. This is something that not even the Shield Weapon Fighter can do, even though it`s been suppositioned that it SHOULD work that way (able to ignore 2WF).
Maybe it`s not the best weapon DPR wise, but you`re getting tons of free Bullrushes, removing enemies` opportunities for Full Attacks, possibly leaving them Prone (if they hit a wall), at provoking AoO`s from ally`s once you get Greaer Bullrush. For that type of build, you can definitely use a Heavy Shield. (and as rkraus points out, heavy shield just means you need to use it as the main-hand and a light weapon as your off-hand)
For a 2WF build (whether 2WF style, or Shield style), you can take advantage of maxing STR and not worrying about DEX requirements, while benefitting from low-level benefits of 2WF.
Try it out. Buy all the armor you can, use the shield, and see how it goes.
You've got, what, a month before you even need a third level feat picked out?
Try fighting in every different way you can think of, try bull rushes, charges, whatever.
Then, come back. The boards will be here.