I have used both and have not had issues with either. Some will say they've had problems with both sites, but that's just like any other company. No one gets 100% great reviews. I normally just choose which site has the deeper discount when I'm ready to buy.
Just keep in mind when ordering to make sure at least 1 penny of your online order is paid for by your Red Card and it is listed as your "default payment" or you will not be able to stack the discounts. For instance, I normally buy $150 at a time so I just purchased a $142.47 card from Raise (remember that the total you're spending on Target.com is 5% less so in this example I only need $142.50 NOT $150). I pay $142.47 with the gift card, and then $0.03 with my Red Card to get the stacked discount.