As pointed out, the only reason to buy either one is because a) you're replacing a broken CPU on your current system, or b) you're looking for a quick upgrade on your existing system & don't have the budget for a full system replacement.
First off, are you 100% sure that both CPUs are supported by your motherboard? Best place to check is on the website. If you're not sure, let us know which model of motherboard you have.
Secondly, if they are supported, it's a toss-up as to whether 1 is "better" than the other. The 6300 has a higher base clock, but is only a 3C/6T CPU; the 8300 has a slightly better max Turbo, & is a 4C/8T CPU, plus has slightly more L1 & L2 cache. However, I would only recommend either one if you're limited to 95W CPUs (which, if you are, I'd recommend the 8320E or 8370E as alternatives to the 6300, as they're also 4C/8T CPUs that are roughly equal to the 8300). If your motherboard can officially support the 125W CPUs, a full-blown 8320/8350/8370 would be a better choice.
Thirdly, a lot of it's going to come down to which CPU your system previously had. If you already had a 6300 or 8300, then it makes sense to replace it with the same CPU. If you had the 4300...I'd skip the 6300 & go for the 8300, as the 6300 is at the same tier as the 4300, while the 8300 is 2 tiers higher (http://www.tomshardware.com/reviews/cpu-hierarchy,4312.html). If you had one of the earlier Bulldozer CPUs (41xx, 61xx, or even 8100), then either CPU is a definite upgrade (although if you're replacing the 8150, you'll need the 8300 or better just to equal it).