We have both, and I'd tend to pronounce "Wirth" similar to "wit" as far as the "i" goes. It's not always clear just from looking at the letter. But some words have explicit cues: There are "stretching consonants" like a-a, a-h, e-e, e-h, i-e, i-h, etc: Aal, Kahn, dehnen, dienen, sühnen, etc. And sometimes the following consonant gets doubled up to indicate a shorter pronunciation, like in "Bitte".
The "i" sound in "wit" does exist in German and is what is normally indicated by "i" on its own. The long "ee" sound is normally spelt as "ie" in German.
And what's the difference? AFAICT it's pretty much exactly the same sound, except in one case it's longer, in the other shorter. Say "bit"... Then say it again, only looonger... And you get "beet". Say "wit", but longer, and you get "wheat".
