I currently do not have enough money to install the header and full cat-back system.
If I were doing one thing at a time and had to choose between installing an S pipe OR a magnaflow muffler which of those would give me a more instant gratification, noticable power gain?

dude... go with the muff...... eeehhhler..... you will get that sound you want.. that feel you love.. and its a better starting point.... just my $.02

yep. something you'll hear will give you instant gratification.