Given that they both are likely to compute the same results, mostly by timing, using %timeit in Jupyter or jost the Python time.time() before and after. The one thing to get right is to do torch.cuda.synchronize() before you start timing and the calculation and after one batch (or whatever) is done.
Now, when you have different amounts of memory and that affects your batch size or one card is fp16-capable and not (e.g. GTX cards are not), you probably want to inlcude that in your estimate.
Best regards
Thomas