Well you are almost asking for opinions based on preference more than on anything else. When it comes to potency there is not a great deal of difference between the two IF they are the Real McCoy’s.
If you have Shantibaba’s real true White Widow, that he renamed Black Widow when he went off on his own, it is fantastic if grown properly. The thing is that many breeders have their own version of White Widow where they crossed the same strains but they were not of the same original genetics so one White Widow will be good or darn good but another can be utterly fantastic, but that is because it is the real deal if it is fantastic.
Joey Weed claims to have the real deal and it is at a very good price and some breeders/seedbanks will not tell you what their genetics are, and usually they are not original, but they will price their gear high because the name alone means people will buy it.
There is nothing at all wrong with White Rhino, again if it is the real deal. Personally I prefer White Russian which is a cross between White Widow and AK-47 but that is only my preference.
The one knock against White Widow, even the real deal White Widow, was its taste. It is potent as heck but is not among the best tasting of strains so that is why so many people used it to make other crosses in hopes of retaining its potency/high level of THC but improving its taste.
If you are going to pick between White Widow and White Rhino if you get the real deals or at least very high quality knockoffs either way you will be happy. I myself would pick the White Widow but then instead of picking between White Widow and White Rhino I would be picking between White Widow and White Russian and that would make the choice more difficult for me.
I grew some Joey Weed White Russian, which is his knockoff and not the original genetics, and the big fan leaves looked like a snow covered mountain they were so white. In Joey Weed’s version of White Russian the father in his cross was White Widow from Greenhouse seeds in Amsterdam. The mother was AK47 from Serious Seeds and while not the original genetics it is still very high quality genetics and when you consider that his price is something like $57.00 U.S. that is one heck of a good value.
When it comes to Joey Weed’s White Widow it is an F2 made from seed stock he got from Green House in Holland in the late 90's shortly after it had won the cannabis cup. Many people have said that the White Widow sold by Green House today is not the same as what they offered years ago so Joey Weed used the seed of the original genetics and sells it in F2 form. Again if I remember right it is like $57.00 and compare that price to the prices of many other breeders who sell their knockoff versions of White Widow that are not from the original genetics and what they charge for their gear. Again a very good value for your dollar with Joey Weed.
With either of your choices you will not be dissatisfied IF you find either the Real MoCoy genetics or a high quality knockoff but then when it comes to knockoffs it can be hard to know for sure if they are high quality unless you know someone who has grown them so there is a degree of gamble to it but flip a coin to pick the strain and then roll the dice for who to buy from and do your best to get the most out of your plants and hope for the best.