Hi!There are thousands of views on how to develop technique...I believe learning many pieces with increasin difficulity, and learning them correctly is the way. Also choosing good pieces is important.To learn more pieces I refer to a post by Bernhard:https://pianoforum.net/smf/index.php/topic,4689.msg44169.html#msg44169I think that will help you to learn many pieces and learn them better.About those pieces you mentioned, I think both are good for technique, but their levels are very different. I dont think you should learn pieces in a much slower than normal tempo, it will not be so fun musically...Hope it helps,Jacob