Oh dear. That's a topic of debate right now actually. Short answer: I don't know anymore.
Apparently, 500mg E7D will have a higher concentration in your blood longer, which
could give you better gains, BUT you will experience more side effects as your estradiol will also spike. A week ago I would have told you that it won't make a difference and to go with E3.5D as it keeps E2 down and allows you to manage it easier, but now I'm not so sure.
I suppose it comes down to how E2 sensitive are you? If you have had gyno or severe acne, I'd probably opt for E3.5D; if you didn't have acne as a teen and not gyno prone, I'd try out E7D. I'd love to see more experiences with a longer duration between injections as we're so used to just telling folks E3.5D.
My .02c