as similar as possible

Yes, they are, and I cannot, for the moment, think of any reason to prefer one more than the other.

When I grow up, I want to marry a gal who is as similar to Mom as possible.

When I grow up, I want to marry a gal who is as similar as possible to Mom.

If there were a number of words starting with S or containing lots of that letter, you might prefer one arrangement over the other, simply to avoid undue sibilance when reading aloud.

When I grow up, I want to marry a gal who is as similar to Sis as possible.

Not only sick, but also rather hissy... I'd change "possible" to "can be".