There are two most obvious differences when it comes to comparing singleplayer aspect of the game and the multiplayer one.
When you play wiith other people, all mobs/champions and bosses deal more damage to you cheap D3 gold (it is confirmed - this was hotfix'd, that change right here changes hell of a lot for us and makes playing barb tank in group a lot more fun, easier and viable) and have more HP. Lets say that you played with your friends all the time until now. Solo play might require you to adapt to completly different type of playstyle in order to be successful in it. As an example, you might think twice before going for pure tank build with very low dps (like 4-5k for example). If you will have such dps but lets say very good survivability buy Diablo 3 items, you still might fail in some situations. Well, lets say that you are able to kill everything. But still, it will take way too long. Diablo 3 is a 'farm' based game (well, you know what I want to say, I didn't mean that d3 is farmville or whatever...) time in it counts along with how fast you can clear stuff. TIME IS MONEY! Make sure that you balance your stats right, I can't stress this enough.
As far as picking of your follower for inferno goes: both the Templar and the Enchantress are viable. It is only that Enchantress is so much better and useful in overall terms. Let me explain why. Templar is great when it comes to saving selling Diablo 3 account your life (if you pick correct abilities for him of course). He might be even better than Enchantress during early levels.
But as far as Inferno and late game goes, you just have to pick Enchantress. She provides you with a passive aura called Powered Armor that increases your armor by 15% (!). On top of that, all your attackers are slowed by 30% for 3 seconds. The best thing about this aura is that it is active even when Enchantress is dead. Lets be honest, companions are kind of derp and tend to die a lot. 100% uptime on 15% increased armor aura is so damn good...