To be honest, only the high end Fender and Marshall Tube amps sound great but they are really costly and high maintenance. The solid state ones from these 2 brands are just regular average amps with a brand name on it. The Line 6 Spider II digital amp has built-in effects and 12 essential different amp models in one single package which includes Marshall, Fender, Vox, Mesa Boogie, Roland Jazz Chorus. Flexibility, convenience and value is what we are talking about.
If you buy a Marshall, you are stuck with a Marshall sound only no matter how you adjust the settings, EQ and drive, it's non negotiable. The same applies for Fender or any other brands.