The mechanism is the indirect effect of E2 on cholesterol. The effect depends on what E2 levels that you achieve and what AI you use to get to a specific E2 levels does not matter.
Letro has unpredictable dose-response characteristics and some males can drive E2 too low with small amounts. Adex has a self limiting dose-response. If you take too much adex, the results may not be different, just a waste of money. It is very rare for a male to have E2 go very low on adex.
If you use letro, you will need to do E2 testing to get the dose right. If libido crashes, E2 is probably too low.
Taking E2 to the levels of lean young men, 15-20, will not harm cholesterol at all.