Cat door research

http://www.homedepot.com/Doors-Windows-Doors-Pet-Doors/h_d1/N-5yc1vZc1i9/R-100281030/h_d2/ProductDisplay?catalogId=10053&langId=-1&storeId=10051 - heavy duty, well insulated 94.00

http://www.homedepot.com/Doors-Windows-Doors-Pet-Doors/h_d1/N-5yc1vZc1i9/R-100404336/h_d2/ProductDisplay?catalogId=10053&langId=-1&storeId=10051 - one flap, magnetic closure like above, 34.00

http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?productId=203046354&storeId=10051&langId=-1&catalogId=10053&MERCH=REC%2d%5f%2dproduct%2d2%2d%5f%2d100404336%2d%5f%2d203046354%2d%5f%2dN - same as last one, but smaller, 5"x7" 30.00

http://www.lowes.com/pd_235667-953-PPDS-L_4294777245__?productId=3365860&Ns=p_product_price|0 - same as above, slightly cheaper at Lowe's, 28.83

http://www.lowes.com/pd_235668-953-PPDM-L_4294777245__?productId=3166241&Ns=p_product_price|0 - same as second down, same price, too, 34.00 

We have an tiny cat and a normal size cat.  I'm wondering if I shouldn't get the next size up just in case a future pet is larger than the current felines.