The reason several categories were suggested is that neither kokopeli nor I did an in-depth review of your site -- that's the job of the submitter and of the reviewing editor. You should look at the contents of your site and decide what is the single most appropriate category, and submit there. Once.
The number of sites waiting for review in a given category is largely irrelevant. If you submit to the wrong category, an editor will just send it to the correct one, where it will wait in the same unreviewed pool as if you had submitted there in the first place. You can't jump the line by submitting to an inappropriate category -- the only thing that is likely to happen is that you actually increase your wait if it takes a long time to be moved to the correct category.
[Added] I noticed that you offer gates (containment), invisible fencing (containment), and doors (arguably containment of a fashion, keeping the wrong kinds of animals out). I'd still say the focus of your site is containment, but I'm not an expert in that category. There are also at least 2 other sites that offer pet doors in the Containment_Systems category.[/Added]