Regarding acceptance it is best not to ask a merchant if they accept but just simply insert the card. Why? Many times in Europe merchants have told me "no we do not accept Diners Club" and this would include Discover but inserting the card in the newer terminals got the card processed. First it got me surprised and after a few successes it is my pleasure to surprise the European merchants... LOL.... I am promoting Discover because when the card comes out of the terminal the merchants always want to see it as they do not recognise the card (Diners is grey) and I use my Garnet Discover IT...it gets instant attraction when processed as Diners Club... I am so glad there is the small DC logo in the back of the card otherwise I might get into jail for presenting a self made card 
Still recommend you bring a MC or VI along during travel but don't be afraid to swipe Discover ...it comes out if really cannot be processed 
And regarding customer service I would say Discover has the lead 
.....There is really only one thing I do not like and hope that will get adjusted in the course of my 2nd year as member...