I do the xrays. I did just do a ultrasound on my French Bulldog. My vet says you can do it at 21 days. I waited until 30 days. We did see 2-3 pups but she couldn't be sure. The ultrasound is not as accurate in my opinion, but you can see much earlier if you can't wait to find out.
both the xray and the ultra cost about $85 in my area. Congrats on your upcoming litter. Your dogs are beautiful.