I used to own nothing but 100 watt plus heads. The "headroom", or amount of clean volume available, is greater with the 100 watt version, as is the bass response of the amp as Joey earlier mentioned.
However, after years of playing higher wattage amps, including a VH4 and Herbert, and possibly as a result of getting older, I am finding that clubs do not wish the volume levels to be high, due to noise ordinances in the neighborhood, which are strictly enforced. Also, when I do concerts, the sound company has always requested that the band not play too loudly with their amps, as miking becomes a hassle and unusually loud amps become almost impossible to mix correctly.
By going to an Einstein 50, I am able to still get all the clean headroom I desire, get the amp to break up at a lower volume, and have way more gain then I would personally ever use. I set my ch two gain at no more then 9:30 and it is plenty for what I am paid to play. I find that when I am able to turn the amp master volumes up louder, that the tone is very pleasing due to the tubes being able to break up at more reasonable volumes then with the 100 watt version, but this is just me. Others may disagree with what I find usable and tell you that the 100 watt head is the way to go. In their case, they have reasons that are just as viable for using a 100 watt Einstein, or other Diezel, as my reasons are for using the 50 watt version.
Therefore, you need to think about how loud you normally use your amps when playing out, and whether the extra volume gained by the 100 watts is needed in your personal situation.
Everyone has different needs when it comes to wattage. For me, I am going to save up for a Schmidt, as I can make more use of the 30 watts in 95% of the situations I play in then I can of the 50 watts I currently use.