I have used both in Florida. Realquest is better at finding sales that might be comparable, IMHO. NDCdata is a solid source of information too. I always enjoyed the link to yahoo maps in NDCdata; a big help in giving a price quote for the job. Both programs easily export data for import into forms software, but the Excel format of the file from Realquest is easier to use in making adjustment calculations. MLS can provide a similar file, so it is a redundant capability. NDCdata has fewer quirks in the way the data is presented, parcel id number being the most glaring example.
My Realquest subscription was $200 for the whole state of Florida (reports individually priced with me agreeing to buy a minimum of $200 worth is the reality of the pricing.) NDCdata was $125 for the whole state of Florida. I think both provide a reasonable value for the price being charged. With IRIS/IMAPP supplied by the Mid-Florida Regional MLS, county web sites and MLS, I finally concluded neither were necessary in slow times like these. If I were going to add one back, it would be NDCdata.