Ignoring ② here, is there any difference in daily usage/meaning? Is one used more often than another?
Both roughly mean medium, moderate, average, but:
中位（ちゅうい） is a typical 漢語 and thus used mainly in technical/scientific/formal contexts. 中くらい is colloquial.
中位（ちゅうい） tends to refer to middle position (in a ranking, hierarchy, etc), while 中くらい tends to refer to physical size/intensity/etc. For example, 中位（ちゅうい）の家 is not something we commonly say, but it sounds like a middle-class family which is not particularly rich nor poor. 中くらいの家 sounds like a physically mid-sized house or a mid-sized family (with, say, 3-4 members). 中位（ちゅうい）の自動車 comes off to me as a car which is not very expensive nor cheap, while 中くらいの自動車 is a car which is not very large nor small. 中位（ちゅうい）のチェスプレーヤー is a chess player ranked roughly in the middle, and 中くらいのチェスプレーヤー would sound odd.
The two words are very different in actual usage.
中位【ちゅうい】 is basically confined to only as "middle" in high (top)—middle—low (bottom) tripartite system often used in academic papers (except for 中位数 "median").
中【ちゅう】くらい (位 rarely written in kanji) has no formal definition and is the common word for "(around) middle/average". Synonyms include 中程【なかほど】, 真【ま】ん中【なか】, 中程度【ちゅうていど】 etc.
If you want to tell "slightly above average" you can say 中くらいより少し上 but 中位より少し上 sounds odd because 中位 represents a broad range by itself so that it doesn't really get along with "slight".