Say you port the E7s. What happens when you want to go a little faster...you're stuck with the E7s you just spent money on.
If you save a little more for the aftermarket heads, you'll have a better foundation for future upgrades.
Porting stock heads was popular when there were few options available. Ford guys used to search junkyards for 69 or 70 Windsor 4bbl heads, or if you had the cash you converted to Cleveland heads.
Nowadays, there are so many aftermarket heads at reasonable prices, it's hard to argue against buying new heads.