well if you can find a good price on a 4-valve i would go for it...
but they are more expensive to build... although by the time you spend the money to rebuild and go PI swap you coulda bought a stock 4V.
A bolt-on 4-valve will outrun a PI swapped bolt-on 2V anyday...
plus as you get money later on, the potential of the engine is greater.
Of course if your gonna build the shortblock of the 2V then it would be a harder decision, more money now, but stronger later.
and if yuou do rebuild what you have stay 2V, its much harder to do a 4V headswap on a 2V, alot of stuff is different.