Stadia tend to be slightly more crowded here in the North, but that's probably because GAA is less present. Sectarianism chased people away and unfortunately the peace process did not bring many of them back to the stadia...
In general I find grounds here slightly better, but enough nice grounds in the EL as well (I think of Cork and Shels mainly, but also Longford and Bohs could have nice arena's -- to the latter two I have one word: ROOF :D)
The level of play: not too much difference, though it is definitely better in the South. Linfield would be the only team up here that would compete for the champions trophy in an all-Ireland league, Glens and Portadown at this moment would not be strong enough.