Critical to any system is the Return on Investment
We at SICK propose a 3 step acid test to decide if RFid is right for you:

1....Do you need to write to the label?
If "Yes" then use RFid

2....Can the reader see the label ( line of sight)?
If "No" then use RFid

3....Is the enviroment too dirty for bar code?
If "Yes" then use RFid

If none of the above point to RFid, it is unlikely that RFid will deliver an ROI for you and a bar code reader will be a better option.

Why 13.56 Mhz (HF)
With all the hype in the trade press its can be quite confusing trying to decide what system is right for what application.
HF or 13.56 MHz offers a shorter reading range which means that we can be more " Directional " with the readers. This is critical with conveyor based systems.
So when we read a tag we know its in front of the interrogator.

We extend the range of the HF readers by putting 2 antenna opposite each other in antiphase, which " Draws " the flux a further distance over the conveyor, and allows tags in different orientations to be read.
HF is particulary suited where products could have high moisture content.

UHF offers a longer reading range so it could read a tag some distance away on another conveyor unless the reader is expertly screened.
Reading codes on a large pallet, for example, lends itself more to UHF since the pallet can be stationary for the read cycle, and has time to wait while the Interrogator "Listens" before talking, to ensure one of the 10 channels is available before it "talks" to the Tag. This listening period can take a while because the interrogators can detect other devices up to 1.8 Km away and with only 10 channels one can see why this would be hard to administer on a real time conveyor running at 1 M /sec.
UHF does not penetrate tissue or water so one must consider the product type before chosing the system.