An intake just by itself is literally worthless. Used with additional mods, such as a cam and head upgrade would make better sense.
My vote is with the Longtube headers.....not the "shorty" variety either, if you want 'bang for the buck' results.
But then again, headers by themselves don't really amount to much improvement, horsepower wise. I know there are those who will swear that there is though. To realize their full potential, they should be included with some other performance mods.
If all you want to do right now, is accelerate faster, I'd suggest installing some higher (numerically) rear end gears......and if you have an automatic, squeeze in a high-stall torque convertor as well.
THEN, you will have something to show for your 'bang for your bucks'.
Headers are a "stand alone" improvement. They will give you significant gains by themselves and even more with other improvements to that area (x pipe/mufflers etc) you'll see mostly torque increases with headers but also a noticable HP inrease (and lets not forget the awesome sound)
Intake manifolds however, require other modifications (usually a cam upgrade) before they realize their true potential. You'll see somthing like a 5-10hp increase with an intake alone (not very noticable) but with a cam that might jump to a 20-30Hp increase.