If you have a fund available in your 401K and the same fund is available through your Roth IRA broker, all other things being equal, which route would you take? Just another scenario I've pondered. . .then I did the math. . .if, when you retire, you'll be in a higher tax bracket that you are now, then buy the Roth fund. If you'll be in a lower bracket, then the 401K is the correct vehicle. If you'll be in the same bracket, it doesn't matter. I just thought it was kinda weird as at least one high-profile "financial guy" always recommends a Roth over a non-matching 401k without ever considering the post-retirement income (and projected tax bracket). . .
Conventional wisdom is that most people should invest in this order:
1 - 401k to match
2 - pay off all high rate/consumer debt
3 - max out Roth
4 - max out 401k
5 - contribute to taxable accounts
It can be hard to figure out what your post-retirement income and tax consequences are going to be. By following the above formula you get to take advantage of the free money of the match and the tax deferred growth of the 401k while also getting the tax free growth of the Roth and giving yourself tax diversity. I think it's important to have accounts with different tax treatments so you have more control over the taxes that you will have to pay in retirement.
With so much being unknown and up for change in the coming years you're just kind of hedging your bets by doing a little of everything.
ETA: as I start getting more conservative in my investments, I would be inclined to keep lower-producing/growth investments and fixed income in my 401k and the higher growth investment in my Roth because of the difference in tax treatment so I guess it would depend on what the fund is before I could say would I put it in my 401k or my Roth.