Can I take the coward's way out and say they're equally good in slightly different ways? EDIT AFTER DIGGING: Actually...can I say Bell is a tiny bit better.
If we're looking purely at raw RUSHING numbers, Murray wins, he's got 1,745 rushing yards to Bell's 1,341, and 12 TDs to Bell's 8.
But if we step back, things start to look a little more even. Murray and Bell had similar yard per carry numbers, Murray with 4.7 and Bell with 4.8. Murray had 90 more carries than Bell, which allowed him to solidly beat out Bell's yardage and TD totals, if we extend Bell's stats across another 90 carries, he ends up with slightly more yardage than Murray and is about 1 touchdown lower with 11 (rounding up) to Murray's 12. So in terms of rushing it's actually pretty close.
Then we look at receiving, and Bell wins that handily. Bell has 774 yards on 77 receptions (10 yards per reception), with 3 TDs. Murray has 395 yards on 54 receptions (7.3 yards per reception), with no TDs. Bell is, at least from this year's sample, the better receiving RB of the two.
Adding up their totals, Murray has 2140 all-purpose yards with 12 TDs while Bell has 2115 all-purpose yards with 11 TDs. Murray has 426 total touches for the year (rushing+receiving) while Bell has 359 total touches.
The final thing to peek at is turnover numbers, and Murray has 5 lost fumbles on the year, while Bell has none. Murray has done well the past 7 weeks, not fumbling once but on the year it weighs a tad against him.
I'm tempted to bring Murray's previous injury history into this, but since he's been healthy this year, I'll let it slide.
Still, all things considered, I give a small nod to Bell. He's been a very solid dual threat this year, and managed a nearly equal all-purpose yardage total with 67 fewer total touches of the ball. Murray's edge in rushing yards comes mainly from his 90 extra touches, and even if we give him some benefit of the doubt, we cannot expect Murray to carry the rock nearly 400 times in succeeding seasons, no RB can maintain that for long, history tells us the human body simply cannot take that kind of punishment across multiple seasons (as of this week, he's carried the ball 78 more times than the next closest RB, McCoy; Bell is 3rd).
In terms of fantasy, obviously, they're both excellent, which should be obvious from their rankings as #1 (Murray) and #2 (Bell) among RBs, and #8 and #10 overall. Neither really can be considered "better" than the other, they're both automatic starts and with only 2 points (ESPN standard scoring) separating them from each other, they're literally .13 fantasy points apart per game.
Still, I think Bell's stats are more secure. Getting around 300 carries and 80 receptions per year is more sustainable year to year than nearly 400 carries a year, plus around 55 receptions. The latter rushing number is likely to deflate to around 300 in succeeding years, which means that while Murray still has great potential, his 2,100+ all-purpose yards will have difficulty eclipsing 2,000 again, while Bell has a solid shot year to year of getting to 2,000.