Derivation of 程【ほど】
The modern reading hodo is a shift from older hoto.
Speculation: This element might be the same hoto that appears as a compounding element in various verbs, apparently related to the adverb potopoto "drippingly", in reference to liquids. These verbs include:
- 迸る【ほとばしる】: "to gush, to surge". Compound of hoto + hashiru, where hoto seems to mean "lots of water"
- 潤びる【ほとびる】: "to swell or soften with water". from older ほとぶ. Compound of hoto + suffix -bu, which appears to be an instance of
/b/
↔ /m/
alternation with suffix -mu, "to become like, to appear like". The hoto element could again be interpreted as "lots of water", in the context of "having" or "full of".
If 程【ほど】 derives from the same hoto, it might be a reference to "an amount (of something)".
Derivation of 位【くらい】
Originally a compound of 座【くら】 ("seat") + 居【ゐ】 ("being, sitting", the 連用形【れんようけい】 or continuative form of verb 居る【ゐる】 "to be or sit in a place").
The initial sense was "where one sits", used then as a euphemism for one's social standing. (For that matter, the English term "standing" is a similar kind of usage.) Over time, this sense of "rank" was extended to refer to various other things.
Usage
My personal sense for the subtleties of word usage is limited as a non-native speaker. My gut feeling is that 程【ほど】 is used in places where English would have "as much (or some other adjective) as" or "so much (or some other adjective) that", as in:
- 「AさんはBさん[ほ]{●}[ど]{●}淋【さみ】しい」, "A is as lonely as B".
- 「Aが高ければ高い[ほ]{●}[ど]{●}、Bが大きい」, literally "if A is high, then as high as it is, B is that big" → "the higher the A, the bigger the B".
- 「どれ[ほ]{●}[ど]{●}食べたのですか?」, literally "You ate as much as which?" → "How much did you eat?"
That said, 位【くらい】 is also used the same way as the last pattern: 「どれ[く]{●}[ら]{●}[い]{●}食べた?」. In fact, 位【くらい】 seems to be more common than 程【ほど】 in this usage. However, 位【くらい】 is never used for the [ADJ]
ければ[ADJ]
ほど construction, and for the first pattern, it requires a の where 程【ほど】 just comes right after the noun.
I'll leave a qualitative analysis of the different uses to other posters.