Crosswind effect on an aerodynamic front wheel vs a rear wheel I am looking to purchase a new wheelset in the not too distant future and am debating between wheels that are 58mm and 85mm deep. 

Or, and this is the part I'm curious about, a hybrid of the two. (58 front, 85 rear)

In a crosswind, a deep front wheel will, I believe, have a larger negative effect on stability than a deep rear wheel.

My question is regarding this. Since I'm going to have one aero wheelset I'd like them to be able to be used in as wide a range of conditions as possible, within reason of course. 

Assuming a moderate crosswind situation, which would add more instability, a 58mm front wheel, or an 85mm rear wheel?
The short answer is that a deep front rim will almost always affect handling more than a deep rear rim. The exact answer depends, as you surmised, on windspeed and yaw angle; however, it also depends on the design and geometry of the bike and the rims in question. For Tri or TT bikes where the rider is positioned maximally forward (or, under UCI regulations, the saddle tip is minimally 5 cm behind the bottom bracket), under moderate wind conditions a deeper rear rim tends to improve the bike's handling compared to a normal shallow box rim. This is not an absolute, however -- for very gusty winds at high yaw angles and very deep or disk wheels, handling can suffer. This is why rear disk wheels (though not deep rear rims) are prohibited on the windy Ironman Kona course. Wheel manufacturers have been spending a great deal of effort in recent years improving the performance of deep rim and disk wheels under non-zero yaw conditions.