First off, I think that they are both great and should be experienced at one time or another. But consider this, in addition to the seating arrangements mentioned by a previous poster, the scifi will be dark and as funny and campy us adults find the old "B" sci fi movies some younger children may afraid of giant killer tomatoes or 50 ft woman. It may be a non issue though with your child. If you do go, avoid a car in the front rows, as you will have to crane your neck to watch the movie clips.
Having said that, 50's is definately more light hearted and your 4 year old may find it funny to hear his/her mom or dad being yelled at for not finishing their vegatables or having their elbows on the table. There are also tv,s near each table showing clips of 50's tv shows,mostly comedys I think. For most older adults though it is very nostalgic themeing. So, my vote would be there but definately do sci fi someday!