I think you should give them a close listen. They both have a very distinctive sonic quality, and actually... it's almost comparing apples and oranges.
I liked the Blofeld because it tends to have a certain cold, gritty sound to it. You could argue that it sounds very digital in some ways, and that's what I liked about it.
If you're playing the keyboards in a funk band, it might not be your thing, if you're into wave/industrial then it might be very fitting. Then of course this is all very generalizing.
Other things you might consider:
if you're on a tight budget: get the Blofeld
if you're an analogue fanboy get the A4
if you think 4 voices are a laugh, get the Blofeld
if you like white, get the Blofeld
if you like things that match your OT, get the A4
if you think toasters should also make coffee, get the Blofeld
and finally, if you're into 1-knob-per-function interfaces get a Prophet 6