Both hotels are convenient. The HI Express is next to the Time Square shopping center in Causeway Bay, with hundreds of great Chinese/Asian restaurants of various regions in the area.

The Prudential is in Jordan, close to the Temple Street night market, and within walking distance (or a short bus ride) down to Tsimshatsui or up to Mong Kok.

I'd also check rates on asiahotels.com, which sometimes have better rates than the hotels themselves or Expedia.

They have the Marriott Courtyard for $695 all-in for the 5 nights 12/10-15. It's less convenient, but is newer and should be nicer than your current choices.

Or $622 all-in for the Eaton. It's close to the Prudential, should be at least as nice, and cheaper than what you're finding. L'Hotel in Tin Hau (just east of Causeway Bay) for $647 is another solid choice.

Also check with the YMCA Salisbury directly. For December, their standard room is $125 + 10% service charge. More for harborview ones.