After lots of research between Snowsilk and Tekplas, I bought a Tekplas whelping box last year and opted for the one that has a side gate opening. I thought it looked more sturdier and well made.
Whether you'll be happy with it will depend on your expectations. I was surprised it didn't come with any instructions and had to struggle to build it in the space I had, I know they have a build video on their website, but paper instructions would have made the process easier. The box just came taped up with small labels to say what the part was.
Once it was built, I was a little disappointed with all the rough edges and little plastic shavings from cutting the pieces to size. All in all, I'd say the Tekplas whelping box is okay but it's not stopped me looking for a better one.
I actually started out with a wooden one which just slotted together when I had a litter, but it could be flat packed when not in use. I actually prefer the design of my old one but only stopped using it because it's made of wood.