I view bond funds in terms of yield, quality and duration. Quality essentially says to beware if you invest in corporate or junk bonds, because their risk of default balances their higher yield. Of course everyone wants higher yield, but the market makes sure to balance yield against other factors.
So let me explain duration so you make an informed choice. Vanguard long-term bond fund has a 15 year duration, while the intermediate-term bond fund has a 6.5 year duration. Duration is the "break even" time for a change in yields / interest rates. For example, if rates go up 0.25% for all bonds, then bond funds go down:
0.25 x -1 x 15 = -3.75% (compared to VBLTX yield of 3.93%)
0.25 x -1 x 6.5 = -1.625% (compared to VBILX yield of 2.82%)
Each fund would pay 0.25% more interest per year, but takes an initial loss. So the intermediate fund recoups it's 1.625% loss over 6.5 years, and the long-term bond fund over 15 years. The problem is that rates / yields are at very low levels compared to history. The Fed wants higher interest rates, for example, and that would hurt bond funds proportional to their duration (assuming all yields go up by the same amount as the Fed rate).
If you chase performance with bond funds, expect to get hurt. For the past ~30 years bond yields have been declining. Again, because of duration that means the bonds experience an increase in value at the moment their payments drop. So for the past 30, 20, 10 years you could claim bonds do better the higher their duration. Yes, but... how will bond funds drop another 6% or so, when they are below 4% now? The future isn't likely to be a repeat of the past 30 years, so don't count on those past 30 years in your prediction of bond performance.
In case duration is confusing, the mechanism is actually people trying to make all bonds look equal. If you have a bond fund earning 5%, and rates decline to 4%, your bond fund looks great! And to make it look like all the 4% bonds, the bond fund will increase in value until the payments are 4% of the new value. That's how duration helps in the calculation: with 10 year duration and a 1% drop in yields, you expect +10% added to the bond fund. And that's why declining yields have make long-term bonds look so good in the past 30 years - they keep getting increases in value. Just don't expect the past 30 years to repeat for the next 30 years.