You've probably ordered it by now, so this may be pointless, but I wouldn't get a used asus prime. first off, the warranty may be short or non existent with a used prime, so if you have any issues you're out of luck. the bad wifi signal on the prime is typically blamed on the aluminum backing, which the tf300 doesn't have.
I would actually consider the plastic backing on the tf300 better in some respects because it's not thermally conductive, meaning you won't have to hold/touch a cold device. the plastic has a better grip than the aluminum overall, and is less prone to damage in a bump/fall.
overall, the tf300 is basically a prime. it has a few minor differences, but nothing huge.
there's only a 100mhz difference in clock speed which you won't even notice, and there's always overclocking. the prime can be overclocked safely to 1.7ghz, so the tf300 should be able to do 1.6ghz at least.
the tf300 has half the brightness as the prime, which will only affect you if you want to view it in direct sunlight. I don't know about you, but I'm generally not going to be using a tablet outside or with the sun glaring on me anyways.
the speakers on the tf300 have no subwoofer...but they're still considered pretty good, and that can be remedied by plugging in headphones of your choice, which, if you want good stereo sound quality, is a better option whether you're using a tf300 or a prime. no one buys a tablet based on the speakers because the speakers in all tablets are lacking compared to even a $20 pair of headphones.
there is no led flash for the back camera on the tf300, which most people find useless anyways. the only way I can see a flash being useful is if you're trying to take a picture in the dark, and that's a limited scenario anyways.
other than what I've stated, there's not too much of a functional difference between the two and anything I might have overlooked isn't worth mentioning anyways. cost and warranty wise, the tf300 is a good buy over a prime. I'd be weary of used tablets because they generally have little to no warranty and/or come with problems. All tablet models have issues, most notably after a while the touch screen can become less responsive, especially from minor scratches to the screen. that issue can range from a minor annoyance to a death sentence if there's no warranty. it'd be no fun to have a section of your screen unresponsive to touch.