Nigma - HP is the horsepower rating on the chiller - an energy indicator of the cooling capacity (and often size as well). More HP = a larger chiller capable of cooling down a larger tank quicker, without the chiller having to stay on as long. The larger the tank the more HP is needed to be able to cool it.
I'm not sure what you mean by the holes in the cabinet? Both the 130 and 250 come with a good size opening in the back of the cabinets for chiller pipes/tubing and/or sump siphon/return pipes to pass through. If your talking about the openings on the back raised rim of the tank. the RSMs do come with those also, to allow chiller pipes to pass through. Some folks do, however, modify things to suit their needs, by cutting out sections of the back plastic lip.