the difference is everything else. for a current metropcs customer the mega will set you back 400 bucks, as will the 16gb nexus 5. i can understand getting the mega if you really need teh big screen, but just about everything else is better on the nexus 5. the bigger battery is subjective, touchwiz drains more battery than aosp, as does a bigger screen, so it all depends on the persons usage. with the nexus 5 it has newer software (means it tends to be faster and less malware), screen resolution is better, chipset is more than double in performance, updates directly from google, its multiband so if metro does something to piss you off you can take it to ANY gsm carrier, heck you can even take it to ting and various other sprint networks if you prefer cdma.
its not that the mega is a bad phone by any means, it just isnt worth 400 bucks IMO. motorola launched teh moto g which has specs that are comparable to the mega in many ways and its only 200 bucks for it. obviously some people are going to be genuinely interested in the mega, just like some people were genuinely interested in the lightray.
why so many people are adamant about posting "negatively" is if someone who doesnt research devices and is looking for the "next big thing" by samsung is gunna see the mega as teh newest device by them and think its the best one. chances are if they are coming to this site they care about rooting and having custom roms etc.