They're rather different, in many ways, and hard to compare straight across. The 598's are open-backed, with a much more airy, bass-light sound, and all the benefits and downsides thereof, and the M50's are a closed studio-style can with a sound signature that's boosted in certain ways which makes it a love/hate can for many. I personally like both headphones (though I no longer own my Senns) but I use/used them for totally different types of audio. I wouldn't call either of them a "Good for every scenario" type of headphone.
If you're trying to pick between them, I'd start with open vs closed. Are you going to ever use this headphone outside/in a public place/around other people at home? Then the 598's probably won't work for you. The M50's also include multiple cables and a bag for easy carrying. However, if you're mostly going to use these at home in a quiet environment, you'll probably prefer the 598, especially if you listen in long marathon sessions. The open backed design and velour pads will make your ears way less sweaty.