...but this isn't me we're talking about, it's my players. I can't force them into it - they need to want to do it. Both are fairly hack n' slash types - which is fine. With that mindset, there are certain powers that seem to stand out (Rain of Steel, for example). So...what we end up seeing is two fighters that look and RP differently (dragonborn vs dwarf) but select many (if not all) of the same powers each time they level.
I don't like the rationale that says "At 11th level it will be different..." because it should be different now. And not just different thematically - but different mechanically.
Well, if we're talking about your players, then you're right, you can't force them to do things differently. With your Rain of Blows example, I can understand the frustration. I also happen to think that power is broken as written, but that's neither here nor there...
There's no "best" path for a Fighter though, as some have different advantage. In my party we have a Pally and myself as the Fighter, as well as a melee Cleric. As such, our front line is usually dealing with 3-4 guys at once. Due to marking and Divine Challenge I'm not usually taking them all on myself, but I'm usually adjacent to a lot of enemies. As such, I was probably going to take Sweeping Blow as my 3rd lvl encounter. It allows me to add in my Str mod because I'm using an Axe (+4, but only +2 for the purposes of the power) and I can mark a whole gaggle of enemies. If I can get the Cleric to set me up with Priest's shield beforehand, and rely on the Pally to take a hit for me if I get in too deep, I can draw a whole lot of enemies to me temporarily and do some AoE damage on them.
So the problem here isn't the system, it's that your players are not being creative. They're just reading the powers list and picking stuff that looks like it will be the most optimal ability, with no thought as to the flavor of their character. Sadly, there's nothing you can do about this except to encourage them to differentiate. I would let them know that building their Fighters to be so similar could lead to some tactical issues because they're both have the same weaknesses. I would also point out that it will cause them to build their guys the same in terms of stats (namely, by going swords and/or flails, and having to pump Dex), which isn't an important stat. If one of them went the Hammer/Axe route, he could pump Con and would act more like a Fighter (Brute) and be able to tank more damage. Which goes nicely with the AoE Fighter powers! The path is especially useful for Dragonborn since their surge value and Breath Weapon damage are all boosted by Con.
To use your example, a 2handed Ftr should do much more damage than a sword and board. Not +1 or +2 with PA. Lots. At the expense of attack roll bonuses and AC, certainly. I think there's a way to balance those two examples without borking the ruleset. In short, we should have a Fighter (Soldier) and a Fighter (Brute) - if I were to use monster roles to describe those archetypes.
WP
As for the whole Greatweapon Fighter versus Guardian Fighter, you need to look at powers as a whole, not just their minor attack bonuses. GW Fighters get Potent Challenge (Two-handed Weapon only: add Con mod to combat challenge attacks) , for example, and they get more mileage out of Power Attack and Reaping Strike. And since Power Attack is a
feat you can actually do a Reaping Strike with Power Attack activated. You would get a -2 to hit, then if you hit you would deal +3 damage, and if you don't then you would deal Str mod on a miss. As for the Guardian, he gets more mileage out of powers like Interposing Shield (Immediate Reaction to adjacent ally being attack. You can add +2 to their AC, or +4 if you have a shield), or something like Shield Push, since he has Tide of Iron.
Additionally, Greatweapon Fighters will tend to gravitate to the Axe and Hammer groups because of the bigger hit dice. Going with Adventurer's Vault items, for example, you could use the Executioner's Axe. It does d12, brutal 2 (so reroll all 1's and 2's). Damage is from 3-12 then, which means an average of 7.5, and it's high crit. Compare that to a sword and board Fighter with a Triple Headed Flail or Bastard Sword. Both are 1d10 damage, which is an average of only 5.5. So just looking at two likely superior weapons you're already going to be concentrating on different stats (Str+Con for the Axe, Str+Dex and Wis for the Heavy Blade), the Executioner's Axe will have an average damage 5 points higher than the Bastard Sword (+3 from PA, and +2 because of the brutal and larger die) and a will have a comparative -3 penalty to hit (Axes are +2 prof, Heavy Blades are +3, and PA will add a -2 penalty, difference of 3 points).
So without adding anything besides Power Attack and the Superior Weapons, you already have the Greatweapon Fighter hitting with a comparative -3, but for 5 higher average damage and the chance for a high crit. Provided they don't take any other feats, this can be accomplished at level 2. Even with just the Greaweapon Fighter taking PA at level 1, and the Sword and Board taking something else, you can still get a version of this if the Greatweapon Fighter uses a Greataxe, and the Sword and Board Fighter uses a Longsword. I don't feel like redoing the calculations, but you can see the point I'm sure.
(ps- You might want to show your players this example. It might show them that's it's pretty easy to create wildly different builds and inspire them to differentiate. With a little leniency in letting them change up stats, that Dragonborn Fighter could dump Dex and pump Con and create a nice Executioner's Axe wielding Greatweapon Fighter capable of inflicting some serious hurt.)
