I have very limited experience using both of these programs ( I interned with a commerical brokerage firm for 3 months) But from what I can remember, it seems that loopnet was easier to work with, searchability etc. Sorry I don't have more to offer, but Good Luck.
I worked for comps.com which was purchased by CoStar. The data collected was limited and undependable. I would use this service only to identify possible comps but I would not rely on the correctness of anything,including whether the sale is arms length.
I have and I use both sources. The utility of LoopNet is going to depend entirely on how much the local brokers use it, because that's where the data comes from. I like the ability to look up current listings for both sales and rentals. None of the AI members in this region are contributing to the database, so that aspect is irrelevant at this point. The main strengths seem to be it's relatively inexpensive and all the information is from the brokers so we don't have to conduct lengthy interviews with them to disclose everything. The big weaknesses are the limited amount of data in relation to the number of transactions, and the menu-driven search modes that force re-entering every parameter every time one parameter is changed (you have to start each search or variation of a search from scratch). That wastes a lot of time. Also, there are only two options for printing out the comps, 1-liners or 1 page; nothing in between.
CoStar COMPS has been around for a long time, but IMO, their quantity and quality of data seems to be declining. I'm seeing regular instances where the information cited on a COMPS entry is incorrect, including wrong photos, wrong building descriptions, and wrong income/expense disclosures. Some of that is probably due to misinformation from the brokers, but some of it is carelessness, too. I see some information disclosed in LoopNet and the local MLS that doesn't make it into COMPS. I've used COMPS for 12 years now and liked the books, which were updated monthly, the best. That was discontinued a few years back (1997 I think) in favor of a CD-ROM distribution, also with monthly updates. I didn't like that at much as the book because you only see what you look for instead of seeing everything, like in the books; good luck if a COMP is categorized incorrectly. However, I liked the CD-ROM distributions much better than the web-based site they use now. It is invariably much slower than the CDs every single time and it wastes a lot more of my time in searches. Ocassionally a server goes down, during which time there is no COMPS. I find it ironic that with each technological step 'forward' they make, my subscription costs are going up even though their cost of delivery and the quality of their service is going down.
Regardless, I use both of these databases, as well as 3 other databases (MLS, NDCData and Acxiom) because no one source has it all. I like knowing that, except for the ocassional pending or just-closed sale, the brokers can't come up with more relevant data for my assignments than I can. Doesn't stop me from asking them to feel free to supply data, though.