In answering questions here on the forums I have both Taber's and Mosby's dictionaries. I've always had a copy of Taber's since I was a student back in 1973. I like Taber's because it has better pictures and explanations. It has more information in the appendices. Also, for nursing it has the complete NANDA taxonomy of the nursing diagnoses in the appendix and Mosby's does not, which when you get into nursing school will be invaluable. I taught medical terminology and I found Taber's to be a better dictionary for finding the definition of word roots.
The best way to learn medical terminology is by using flash cards to memorize the roots, suffixes and prefixes. Trust me.