The FCC and FDA designated two portions of spectrum for wireless telemetry, WMTS (wireless medical telemetry service) and ISM (the Industrial, Scientific and Medical band). Since the inception of WMTS in June of 2000, and the release of WMTS and ISM based medical devices, there has been a lot of talk about one band being more "protected" from interference than the other.
There are two kinds of RF interference, intentional, and unintentional. In the previous string, the talk has been about intentional interference from HDTV, TV, and PLMR (but NOT ham radios).
By far the most frequent (and troublesome) form of interference is unintentional. Bad fluorescent light ballasts, elevator motors, microwave ovens and other RF radiators in and around hospitals.
Vendors have made a lot of hay about how WMTS is "protected." It is protected, but only from intentional interference. And even then, you must identify the source and go through a formal process to resolve the situation. This could take weeks or months to resolve.
WMTS is a designation of frequency ranges, a piece of RF real estate. Vendors can develop any kind of communications schemes within that band that they want. There are no standards built into WMTS to ensure coexistence, maximize capacity or minimize the impact of interference, intentional or otherwise.
The ISM band (approved by the FDA and FCC for medical telemetry) is also a piece of RF real estate, but it includes standards that everyone must follow to ensure (or at least facilitate) coexistence, enhance capacity and minimize the effect of interference (either power level restrictions or protocols like 802.11a/b/g). This is why GE (and soon Philips) is employing ISM-like protocols (spread spectrum frequency hopping) in their WMTS products.
ISM has many times more bandwidth than WMTS, thus providing greater theoretical capacity, and more room to avoid interference. In larger facilities, this may be an issue. Also, WMTS coexistence between two or more vendors is a big issue. I've seen a GE system hop all over a channelized Philips system, rendering the Philips system ineffective. WMTS infrastructure, i.e., access points, are proprietary -- although the network cable pulled to them is not.
In the real world of patient monitoring and surveillance, WMTS and ISM are substantially equivalent. Which is best depends on the details of your particular situation, and the vendor(s) involved.
The real criteria come into product features like scalability, reliability, ease of use, and support for your clinical workflow. Oh ya, and the devices that hang off the network.