This question is probably applicable to most modern coins but for illustration purposes I'll use a Roosevelt dime, the 1996-W. For the long term, 10 - 20 years, would it be better today to purchase a PCGS 1996-w MS67FB or a PCGS 1996-w MS68? Let's also say that the price difference is negligible and FB and FT have the same meaning. Would more weight be put on the FB/FT or on the slightly higher grade thus making one more valuable 20 years from now.
I don't think in 20 years there will be a dimes worth of difference I don't think in 20 years there will be a dimes worth of difference in the 2. (no punn intended)! They are all from mint sets and most are very nice if forced to chose I would go for the MS-68 but I would probablly go one step further and get a a 69 if I wanted to have one. Or save a bunch of money and just get one in mint set package and forget it.
The requires guessing, of course, but I'd go with the FB. I think there might be more collectors trying to get the FB designation than there are collectors who care whether it is a 67 or 68.
Not sure the price of them in those grades, but if you think the value of these will increase long term because of it's a 1996-W, then why not just buy a bunch of 1996 mint sets? I'm a big fan of the 1996-W dime as well and I buy '96 mint sets every time I can find them at or below greysheet bid as long as the dime looks nice. Probably had over 50 sets now, most of them acquired for around $10 each. I think if this coin appreciates in the future, I'll see a greater percentage return on my $10 1996 mint set than someone will on a higher cost slabbed example (unless it's a top pop coin).
I fear that what I am about to do is mostly reveal my rather jaundiced opinion of moderns, but in response to the original poster's question, I honestly think that the so-called "choice" is a difference without a distinction. The mintages of these are so huge that it is improbable that either will either appreciate much in 20 years. That's the problem with modern "mint products," imo. Supply just overwhelms demand.