You haven't provided enough information.

1) Climbing with full power

What Speed? The pilot selects the AOA when he chooses to either climb
out at Vx, Vy, or a Cruise Climb. Of these, Vx is the closest to the
stall, so lets pick that one for the sake of your question.

2) Descending fast with 50% power.

Again, what type of descent? Straight forward and clean? Emergency
Dirty? Because Of lack of specificity, I am going to assume you mean
straight forward and clean, and as such, I'm assuming you mean a high-
speed (cruise) descent.

Both have constant rates of ascent and descent.

So to clarify, your assuming that all 'zooming' energy has been
expended and the aircraft are in a state of equilibrium?

Also, given that you have not specified a particular aircraft, I am
going to use my own personal Bird's speed's as a reference for all
further questions (PA-28-140).

Question A: Which wing is closer to stall?

In state 1, the ship is operating at 74mph, on the low side of L/D max
operating in the region of reversed control authority.
In state 2, the ship is operating at its parasitic-Drag-Limited
descent-rate. Lets assume that the pilot is descending at 500fpm
(glide rate, power off). Obviously the pilot is going to be doing a
lot faster than the 80mph Vg, since the plane can do that power off,
and in this case the pilot will be suppling a substantial amount of
energy from the mill... in my bird, 50% power on a cruise descent
gives me ~140mph, but in any case you'll be substantially faster than
the best glide speed.

That said, the answer is trivial, of course the 74tas aircraft is
closer to stall than the 80+tas aircraft.

Granted, there is an odd case, its possible that the pilot is
operating at 50% power, but right ahead of the stall regime in the
region of reversed command authority. In that case, if he is
requiring 50% of his power to maintain a 500fpm descent, he's probably
pretty damn close to the wing's stall threshold... But that would just
be poor judgement on the pilot's part.

Question B: A big updraft occurs, which is more likely to stall?

I'm not sure Updrafts are your problem so much as tail-winds, but in
any case, obviously the ship that is operating closer to Stall will
have less margin for variances an ambient wind direction.

If you vote first it _may_ lead to some good discussion later...