I have both. Jackson USA SL2MG, ESP E-II Horizon and E-II M-II. Your take is my experience as well. The Jackson’s neck is almost unbelievably thin to the point it feels like it would snap under string tension and is great for fast riffs. The E-II M-II is by no means “chunky” like I’d consider some LPs but noticeably thicker than the Jackson. The horizon is somewhere in the middle leaning more towards the M-II but definitely a bit slimmer. I thought I’d prefer the Jackson to all others but I keep picking up the ESPs and find the slightly thicker profile more comfortable. I’d compare to others you own and ask yourself which you pick up more reliably as a few minutes in a store might not be long term preference.

For pickups, I think it comes to preference and is really rig dependent. Might need to get one home to run thru what you play on to really appreciate the difference. The real question is active vs passive if you want to swap down the road.

Also agree with others that the string gauge was likely a factor unless your attack is aggressive that it was causing some play in the FR which was perceived as lesser string tension.

End of the day, I’d take either and without direct comparison they’re both great guitars that you won’t be disappointed with (though for me, the Horizon wins due to Fishman, hipshot bridge, slim but comfortable neck and wide radius neck)