AT&T or T-Mobile Tourist Plan for CA Coast

I just spend about a month along the CA coast and the T-Mobile service was terrible. Luckily I also have AT&T for data service and that was usable but the signal strengths pretty low. I have been in those same areas in the past with Verizon and have had no problems. I just had to drop Verizon due to the high cost and low limits on their unlimited data plans.
I am pretty sure that both T-Mobile and AT&T would have benefited dramatically from an external cellular antenna.