All depends what your style is and how you want your power to come out. I think 400hp with a paxton will equate to about the same cost in N/A for 400hp worth of upgrades. From what I have seen so far this seems to be true any way.
The charger would most likely have more torque, but since you spent all your $$ on the charger your redline would still be around 5.5k. More streetable for sure. For the N/A the same amount of money would probably be spent on H/I/C which probably would set you in the 6k-7k range for redline and most likely would shift your powerband up the rpm scale a notch or two, all dependant on what cam you use of course.
I think the N/A route is better though. If you go with a completely new block, seeing as you can order it with those performance parts already, whereas it is hard to find someone that will sell you a stock block with a paxton on it, alot of money can be saved getting you to the 400hp ish mark in about 3k and some. One would have to get the stock block and then by the paxton. I am not sure how much a paxton or K&B costs but I will guess atleast 3k, which would make the upgrade kind of crazy to me, as you would still be stuck with a used engine or have to shell out alot of $$$ for a new one in addition to the super charger.
EDIT: Just noted you said a mild 302 with a charger on it. HP wise I don't know what your shooting for, so I can't really say much. But it will most likely cost more then the N/A route up to about the 400/450 mark, after that go with the supercharger, but beefed up internals should be come mandatory.