IIRC there was a lot of planning, debate, and negotiation behind the scenes that went into the Diana decision, a lot of it far out of Charles' hands. On his own accord I think he'd have married Camilla from the outset.
Anywho...certainly any minor change of circumstances may, IMO, have led to a different cousin chosen as Princess Bride.
In that case, who knows. Could work out better for Charles than OTL, but maybe not. The ATL Princess may get along even worse with greater and juicier scandal for the tabloids. Or maybe it's Storybook Dreamy Sunshine Land, who the hell can guess?
Diana OTOH may never gain any notable fame, but may have an interesting career in clothing, modeling, acting, or something. She certainly is much less likely to be the eternal target of paparratsi (sp?) of doom.