Criteria for choosing undergraduate school to prepare for graduate study: "focus on undergrads" & "extensive rigorous curriculum" mutually exclusive?
You'll need to decide for yourself which sort of school is the best fit for you educationally (this is a highly personal decision, and a school that's a wonderful fit for one person might drive another crazy).

However, I don't think a focus on undergraduates matters much regarding preparation and admission for graduate school.  Not even in the ways you'd think it should, like getting good letters of recommendation: there are disproportionate numbers of students in top graduate programs who attended institutions nobody would describe as focused on undergraduates, so this is clearly not keeping them from being admitted.  In particular, faculty at universities with strong math programs know what it takes to be admitted to top graduate schools, and they provide what's needed.  You might have to put in some effort to make sure you don't fall through the cracks, and this could be a hardship depending on your personality, but for many people it's not so difficult and in any case it's a valuable life skill to develop.

If you're looking to judge universities by how well they will prepare you for graduate school, the main factor I'd look at is how many students they send to graduate programs you might like to attend.  If the number is large (relative to the size of the undergraduate program, of course), then that's a very good sign.  Clearly they're doing something right, and you'll have the benefits of a strong peer group already as a undergraduate.  If the number is small, then you should worry.