Automatic Vs Manual Transmission

Hi everyone, I'm new to the forum and I may be purchasing a used mustang soon and I can't decide on whether or not to get one with a manual, or automatic transmission. Finding a good car will not be a problem because there are many low-mileage 05-07 v6's at good prices in my area. My question is how are people's experiances with both of the transmissions? Is there a big difference in reliability? Does one handle the extra horsepower and torque produced from modifications better than the other? What are the most popular modifications to each of the transmissions?

Thanks, I thought about getting a GT, but because of my age insurance will be incredibly expensive with a V8. Will getting a v6's horsepower into the upper 300s put enough stress on the transmission for it to be a problem?