1hand and shield pros
-you get a shield
-ability to have a weapon and cast a spell
-bash moves
-moar enchants!
cons
-less damage?
-spreading perks to moer trees
dualwield pros
-more damage
-ability to have a weapon and cast a spell
-moar enchants!
cons
-no block
-no bash
2handed pros
-more damage (not as much as dualwield)
-block (not as well as shield)
-bash
-reach? (undetermined if its actually useful)
cons
-block doesnt block all damage
-slower attack
-less enchants
now mechanicwise skyrim is NOT a precise action game. the combat is floaty and while better then oblivion, still sloppy. This means to me that dualwield might be out from the get go as its a high risk high reward playstyle that requires a finer tuned gameplay.
twohand is a enigma...its a middleground of damage and blocking. but is middle ground really worthwhile? you can stagger easier...but there are stagger immune mobs. you can block...but apparently not block all damage (does the block perk change this?) the range seems moot as there isnt a lot of depth of field upclose in this game.
then from a min/max perspective (even tho in a single player game i hate doing this) a breton with a shield can be 100% magic immune if perked correctly.
its a hard choice, but one i feel probably does have a best choice scenario. this is all the info i know, feel free to add more and ill edit the post when i get time.