Though, as you mentioned, these are two different game types,indeed i'm having more fun in HoTS than in WoW PvP nowadays.The question is - what kind of WoW PvP do you mean?Casual random bg grind and 2 vs 2?Then of course HoTS PvP is way more better than in WoW,but as I heard,the 3 vs 3 is quite balanced,though I haven't tried it this expansion.Yet,if you're forced to roll other class only because it does the same as yours only better,it pisses me off and I won't reroll in spite of it - instead of it I just won't run in PvP at all (for example,the warrior they turned into dull meat with low survivability nowadays). To be honest,lately I play HoTS more than WoW - the main thing I do nowadays in WoW is running mythic dungeons,since I see no point in running HFC normal or even heroic,with typical pug requirements - 730 ilvl and Archimonde mythic achievement.When i'm done with mythic dungeons,I just don't know what to do,since I find the latest pvp season quite pointless,with it's +10 ilvl increase on s3 gear,which yet again reminds of how much Blizzard cares about the PvP,yet again giving Alliance and Horde the same model recoloured gear,which is like a spit in the WoW PvP player faces.
It's worth mentioning,that in HoTS there're a lot of people favourite heroes and the idea,that you can play as your favourite hero - well,it's just great.