Since you already asked for an extension and they declined I would take my decision by April 15th if I were you. The Academic world is small and the April 15th deadline is taken very seriously in the US. Put pressure on you, drink coffee and even don't sleep but make your decision before April 15th midnight. I am nearly in the same situation...I just received the PhD offer 2 days ago (very unexpected) and have interview abroad on Tuesday for a job in a reputed environment. For the PhD I only had one short Skype interview in January with one Professor...then no news until this email which asked me to decide withing 1 week (a simple letter offer with 1-year assistantship). And on the side of that I have a nice supervisor from a highly ranked university that I am in contact with since nearly one year and who is searching for alternative options for me (I got the PhD offer but no funding so maybe I could get a job and re-apply).
I think you have the chance to have the choice...and there is something really simple to know which one to choose. For me for example, I hoped I would get offer from Uni A and Uni B...but in the end just got offer from Uni A. And today I realize I really preferred Uni B over Uni A.
Here is how I would do:
1) Consider you ONLY received the offer from the US --> would you accept it right away ? YES/NO
2) Consider you ONLY received the offer from Europe --> would you accept it right away ? YES/NO
--> if the answers are different then your choice is already made, go for the one you would have taken. If now the answers are YES in both cases think about the differences between the US/European PhD (the length of the PhD is a major point, relocation cost etc) and the confidence you have in your supervisor. This last point is very important. Personally I exchanged many many emails with Uni B supervisor and department staff during many months, I went there to pass interview, chat with lots of people and I can tell you it really makes the difference...for Uni A I only had a 20 min short Skype interview with one person, that's it. So today I am unsure about what to do but I will have to take a decision as well by Wednesday. My job interview is on Tuesday so I hope they will tell me directly what.