Stacks have a 4x12 closed back speaker cabinet pushing allot of air.This makes a good sound if thats what your after.Its a bigger tighter sound with more power and punch.The disadvantage is there is not any places big enough to crank a 100watt Marshall.Back in the days of Jimi and his cranked 100watt plexis,was also the days of tiny PAs and stage volume.Allot of guys think the pros are cranking there amps on stage,um no.I played concerts befor and a 50watt Marshall on 5 is plenty loud for a small theatre,7 is bordering taking control from the sound man.
On the other hand running a say,30w tube amp on stage with say 2 12s has its advantage too cause in most concert venues you can crank the little basterd on 10 and drive the power tubes into orbit.
What makes the differance between the 2 is the cabinets a 4x12 closed back cab simply pushs more air making a tighter,in your face sound.
Also the diferance in wattage makes a differance too.You can crank a 30watt amp and get full bore power tube saturation.But on the other hand a 100watt Marshall can't be cranked in most places but but has an inherant fullness that the little amps just can't give.A/B a 50watt Marshall to a 100w Marshall and you'll hear a bigg differance in body.