I think the only real difference between the MR5 and MR6 is purely cosmetic.
I haven't seen any place that has the MR6 with an egrip (I admit I didn't look too hard) but you can purchase an egrip to slap on it.
Refurbished MR6's are probably returns sent back to the manufacture within the manufacture warranty time frame. Could be a minor hickup that got repaired or the customer didn't like it.
You may not find too many reviews on the MR6 as it did come out this year and it is very similar to the MR5.
I personally would snag up the MR5 with the egrip as I like the looks better and egrips can be set to different modes as needed. The price difference is primarily due to the egrip being already installed.
+14 years of paintball experience.
Loves double chocolate cookies.