Personally i think the MG50s have inferior tone for a marshall and you're paying more for the name then the tone. The line 6 is digital and i hear the spider is hit or miss at times but its good for a nu-metalish tone. If you want to look at alot of different amp reviews you should check out
http://www.harmony-central.com/User_Reviews/ and read some of the reviews there. Personally i would recommend the mesa boogie f-50. Its also 50 watts but man does it it pack a punch and you can get a sweet sweet clean tone and a killer heavy sound. Check it out and i hope you find what you're looking for.