I am a huge fan of the Willy Post but you have to be aware of the rather idiosyncratic cut: high rise, double pleads, drop crotch, cinch in back, strong taper with very loose thighs, slightly cropped.
What it can do to some physiques is to cause diaper butt due to the amount of fabric in top block. If you are on the skinny side with notably absent gluteal muscles, this might not be the pant for you. If it fits, it fits beautifully. And it can be easily worn dressed up with a jacket or dressed down, given that drop crotch/ taper combo gives it a very casual look.
Cinch is a more traditional: It is EG's slimmest pant and ~1 - 1.5 inch narrower in thigh. Less aggressive taper, no drop crotch, flat front. To me, it is the most "formal" EG pant and perfect with a jacket.
Pics above really don't do much justice to the difference of the pants ...
The above is undoubtedly true though I really dislike how much EG has shortened and tapered the Cinch Pant over the past few seasons. I was looking at the Bureau's S/S items and I wanted to buy the grey broken chambray Cinch Pant, but the size 34 (my size) has a 29.5" inseam and a 6.75" hem width. As someone who is 6' tall, this is too tapered and short--like, almost Thom Browne style. Even letting the cuff out wouldn't help the width which, in the end, is the deal breaker for me.