I used Irish moss for many years before switching to whirlfloc. But I think that was an availability thing more than anything else.
PRO- Works great everytime.
PRO- easily scaleable to any size batch
CON- Need to measure it out
PRO- Works great everytime. I found it was even mildly better that IM, but that's tough to call.
PRO- Tablet is great and no measuring required, 5 gallon/s per tab.
CON- Tablet doesn't easily allow for a non-5gallon interval brew (say you did 8 gallon batches or a 3 gallon batch) This is trivial, but you could in theory over do it if you were doing small batches.
CON- More expensive than IM, but we're talking nickels not dollars.
In my experience whirlfloc is easier to use, with few draw backs. But in real world application the two are practically the same performance wise.