Multiple MHA is always fun. Just generally avoid duplicating units (there is exceptions such as stormfall archers).
Invictors are very good shooters and decent melee, they're rather expensive pointswise however 8/12 for 6/10 models and
UA (which they need). Highly accurate shooting with the
UA. And I agree they look cooler than the Sentinels.
Sentinels have no range but fantastic melee. They have good
MAT (usually hit 'normal' targets), Reach (2 inch melee range rather 0.5 inch) which is great, and are weapon masters (roll 3 dice damage instead of 2, if charge 4) which just wreaks everything.
I recommended Sentinels because they are a melee threat to complement your MHSF and they can make good use of Kaelyssa's feat. And they wreak things with much armour which the MHSF can struggle with.
But I want to note that Sentinels aren't straight up better than the Invictors, it all depends on the rest of the list. Sentinels are more popular because they fulfil the role destroying high armour targets which the rest of the retribution can struggle with.
Another option is the Houseguard Halberdiers, 9 pts for full unit with
UA, +2 pts for the Houseguard Thane solo. Quite hard hitting on one turn thanks to a mini-feat from the
UA. They are also faster than the Sentinels, especially with the Thane solo.
Couple of helpful links:
BattleCollege gives you some ideas about how models function.
http://battlecollege.wikispaces.com/mkiiRetribution The official retribution subforum have also written down the conventional wisedom on models.
http://privateerpressforums.com/showthread.php?16513-COUNCIL-OF-THE-NINE-VOICES-Overview-of-Retribution-of-Scyrah-Models-and-Tactics ForwardKommander is a list builder, gives you points for everything and (implicitly) its Field Allowance (i.e. how many of a model you can get)
http://www.forwardkommander.com/
