Which area for calf implant do you recommend? Subfascial or under the muscle?

I have been performing calf implants in the subfascial plane for over 18 years and I am yet to hear of any reputable and experienced calf implant surgeon successfully place these implants submuscular without any major complication. There simply is not a good tissue plan to put these implants under or even within the muscle. I would make sure to get more details from the surgeon and how many he has performed this way and what the before and after results look like. 

Definitely under the fascia. 
However, some people say "under the muscle" when they actually mean "under the fascia of the muscle". 
There is almost no benefit from putting them under the actual muscle, which causes too much trauma and complications. 

I place my calf implants between fascia and muscles - this is much safer position that under the gastrocnemius muscles. Requires much less dissection (trauma) and much less chance of complications. Visibility is usually not a problem if one uses soft silicone implants