Here is a great article written by Bruce Egnater on loops. This may help you decide which to use.
""EFFECTS LOOPS OFTEN GET a bad rap for “sucking tone.” In reality, most guitar and bass amp effects loops are pretty good. A basic understanding of the uses and limitations of the different types of loops can help reduce the frustration. Here are some simple guidelines for using the two most common types of loops.
Series Loop
How it works: Patching an effects unit into the loop’s send and return jacks interrupts the signal path and 100 percent of the signal is routed through the effect. This means that the effects unit should be essentially transparent, and not mess with your tone. Series loops are also typically line level rather than guitar/instrument level.
Optimizing performance: The input and output levels must be properly set for maximum headroom and lowest noise. Here’s how that’s done:
•Set your amp/preamp volume controls for normal playing levels.
•Connect a high quality shielded cable from the series send jack to the effect input.
•Adjust the effects unit input level to “just peak” while playing your most aggressive licks.
•Connect another high quality shielded cable from the effect output jack to the effects return jack on the amp.
•Adjust the effects unit output level to match the volume you heard before connecting the return cable. This is called unity gain.
Advantages: Works fine with just about any effect. No special requirements other than unity gain setting.
Disadvantages: The entire signal passes through the effects unit, which may mess with your tone.
Parallel Loop
How it works: This type of loop operates like the effects buss on a mixing console. It maintains your dry signal, while allowing you to blend in as much of the wet/effects signal as you like using a wet/dry mix control. You cannot use effects that require 100-percent processing, such as equalizers, noise reduction, tremolo, or compressors.
Optimizing performance: You must set the effects unit for 100-percent wet signal, because the dry output signal of many effects units is out of phase with the input signal, and mixing the out-of-phase signal back in with the dry signal can seriously mess with your tone. Follow these steps:
•Optimize input/output levels following the first four steps described above.
•Set the effects unit for 100-percent wet signal.
•Use the parallel loop wet/dry mix control to blend in the desired level of effects.
Advantages Doesn’t mess with your tone. The effects level is easily adjusted. Works well with time-based effects (echo, reverb, chorus, etc.), which work in parallel with your direct signal.
Disadvantages Doesn’t work well with effects that require 100-percent processing. May require reprogramming your effects unit for 100-percent wet mix.
Special Note
Most multi-effects processors have a combination of many different types of effects. This means when using the parallel loop, you must be aware of which effects can and can’t be used. For ease of operation, I recommend using the series loop if you intend to use a mix of different time-based and non-time-based effects in one unit."""
