Fundamental difference between t3 and t3a EC2 instances
Per benchmarks at photographerstechsupport.com, t3 instances tend to be 10 to 20% faster than their equivalently spec'd t3a counterparts. I notice that the precision of some of the benchmark values is low enough (i.e. 0.06 vs 0.07, reported only to the nearest hundredth) that the actual difference could be anywhere between ~0 and 26%. However, based on multiple values it does seem the difference is about 15% overall.
This is reasonably close to the price difference of ~9%, making either choice a good option, depending on the compute performance need.