RA in a lab before applying for phd

[herewego]

Dear all,

I am graduating currently in Engineering and applied maths (master of eng and MS). I am offered a RA (three months at least) at a Max Planck Institute, in a field related to my research interests and with funding. I plan to apply for graduate program in CS and applied maths at ETHZ and in the US in the fall. What in your mind would be most rewarding as a gap year: lab research (with no publication in such short period) or further master studies in France (a program in research in CS and in biocomp are appealing, with 6 months courses and 6 months research thesis), or even R&D professional exp?

Would you encourage working as a RA outside an academic program? Have you had such similar experience? Are US universities open to people with one year professional exp; in R&D or would it be a deterrent?

Another poorly related question: what are good maths subject GRE scores? What was your study material and your grades (and where were you accepted)?

Regard,

H.