The industrial in me as a potter (as in throwing pottery on a wheel, firing it, glazing the piece, & firing again before selling) believes glass is always best as the dark colors of most glass bottles protect the liquid the best; however, the consumer and ease of use person in me likes plastic to take out as it's easier for me to drip, esp since I'm legally blind and droppers in glass are more difficult to feel as I load my e-cig... I'm going to have to abstain from the voting; HOWEVER!!!!!!
If it has MFS e-liquid in it, personally, I don't care what it comes in.

You can send it out in those little milk cartons school children use and I'd be a happy camper - again, as long as there was MFS liquid inside it. LOL!!!!
Please forgive my facetious liberties here but in all honesty, after finding MFS liquids along with the terrific customer service among the staff... I could not think it could get any better... although, I am looking forward to the days when we, like Star Trek, can beam the product to a customer's home. Now THAT WOULD BE COOL! ROFLOL!
I'm just more interested in the liquid and if glass preserves it best or makes your job easier then by all means do what you must. You will still have a customer right here. I'm not going anywhere as long as you guys are around and believe me when I say I will beg, plead, and probably lay on your floor and cry if you ever thought of going out of business. There simply is NO e-liquid in as many flavors (despite my undying love of a few flavors I purchase on a regular basis) available anywhere else and there's no customer service that is as great as anyone else.
So that's the best answer I can give you. I'm sure it probably does nothing to help with your decision but it is as honest and as true as I can be so I hope that accounts for something somehow along the line.
All my best wishes to everyone at MFS,
Paise