Warband 1
Erkenbrand on horse
4 Rohan Royal guard on horse
6 westfold red shields
223

Warband 2
Rohan Captain with heavy armour and shield
12 Warriors of rohan with shield
139

Warband 3
Rohan Captain with heavy armour and shield
12 Warriors of rohan with shield
139

Warband 4
Rohan Captain with heavy armour and shield
11 Warriors of rohan with shield
133

Warband 5
Rohan Captain with heavy armour, shield, horse and bow
9 Rohan Outriders on horse
117

total: 751pts

might: 11
models: 59
break on 30


Alternative 750pts (breaking the fluff)


Warband 1
Erkenbrand
11 Rohan warriors with shield
142

warband 2
Theodred with shield
10 Rohan warriors with shield
145

Warband 3
Eomer, knight of Pellenor
2 sons of Eorl
6 Westfold red shields
243

Warband 4
Eorl the Young
2 Sons of eorl
6 westfold red shields
218

total 748pts

Might: 12
models: 41
break on 21

I would take the first list with some minor tweaks.
I would dump some captains, and take eowyn and gamling on a horse.
Or maybe upgrade gamling to a 3 might hero if you have got the points.
And change 1/3th of the normal warriors to bows.
Gives your archery a little push if you need it, and hasn't really got a disadvantage as defence 4 or 5 most of the time doesn't matter.

And the more heroes on a horse, the better.
Because you will want to call heroic move every time you haven't got priority.
And it is just easier to be in the right place when you have got a horse :p