Well, acoustic guitar is harder work because you've got greater string tension, typically thicker strings and a higher action to deal with, so you'll build strength in your fingers faster. I think you should start playing whichever fits the style of music you aspire to... you need to be inspired, and playing Slayer on an acoustic might not do it for you...

If you play electric only, the effort needed on an acoustic will surprise you... As an infrequent acoustic player, it does me...