HR24 and H24 are the latest models. They have DECA built in so they work without extra dongles for the whole home DVR setup, and tend to be a little faster. Most new installs with Whole Home DVR service will get these, since it makes for a much easier install.
That being said there is no gaurantee that they are what you will get. You could get anything from an HR20 through an HR24, or H21 through H24, as they will all work with Whole Home DVR. You just need to have a small external box at each unit if they aren't 24s. Do not let them install HR20s on your 3D TVs, as they are not 3D compatible. The H21 and up, or HR21 and up are all 3D compatible.
One thing you do need to make sure of, though is the Cinema Connection Kit. If you plan on wanting to access DirecTVs On Demand service you will want to make sure it is included in your order. It is an extra little box that hooks your DirecTV coax network up to your home internet network, so all of you DirecTV receivers can access the internet to download the On Demand content. It also allows you to use things like Apps built into the boxes, order PPV with your remote, stream videos, music and pictures from your computer to your receiver, and watch your DVR recordings on your computer if you install the DirecTV2PC application.