I'm not in the Chicago area, but just from 
looking around, I don't think either convention 
will be a good prospect for cels. If you're 
planning on going just for the cels, remember 
that most cel dealers who attend conventions have 
an online presence as well... It's generally not 
going to work out in your favor if you go through 
the trouble of paying for registration, parking, 
possibly a hotel room, etc, just because of cel 
shopping. ^_~ So make note of the vendors who are 
listed as exhibitors, check their sites, and see 
if they sell cels from the series you're 
interested in. Otherwise, if I had to choose one 
convention over another due to funds, I'd 
probably pick the convention based upon guests, 
programming, and so on. ^_^

AnimeReactor seems to be a second-year con, if 
I'm not mistaken? Generally, younger cons tend to 
have lower attendances and smaller dealers' 
rooms. In cases where small cons have tried to 
enlarge dealers' rooms to give a better variety 
for the attendees, it ends up hurting the vendors 
because there isn't enough of a market to make 
the trip profitable. Cels tend to be a niche 
market, so that's one of the reasons why you 
might not have luck at either. I don't know about 
the group that puts on Anime Reactor, but I do 
know the art show is run by Sollitaire, which I 
believe is part of Phoenix Entertainment, which I 
believe is what's behind A-kon in Texas. So if 
you have a general interest in fantasy/anime art 
in general, not just production art, I'm sure 
that the art show will be run quite 
professionally. But due to the size, I'm not sure 
about the diversity of the artists who will 
either be in attendance or choose to mail in...

Anime Sound & Vision appears to be a more 
corporate event. Japanimation City appears to be 
a major sponsor, if not the driving force behind 
it. This seems to be AS&V's first year. Due to 
its first-year status, it seems iffy that cel 
dealers will take a chance on such a new 
convention---- especially when Anime Reactor 
happened just a few weeks previously. Generally, 
when an area shares two conventions, they try to 
put as much space as possible between them... 
Memorial Day versus Labor Day, and so on. This 
prevents the con-attending audience to recover 
from the last con, and get enthused about 
attending the next one... and replenish their 
supply of disposable income while they're at 
it. ;o) The positive point in AS&V's favor is the 
fact that because Japanimation City is such a 
major sponsor, you know there are going to be 
cels for sale, since Japanimation City sells... 
cels! If Japanimation City is a cel vendor you 
like, you might choose this one over the other. 
However, if they don't provide the kind of cels 
you like, or price their cels the way you prefer, 
or suchlike, it's doubtful there is going to be 
another cel vendor present.

Keep checking Anime Reactor's website 
under "exhibitors". Usually, conventions will 
list vendors who will be in attendance, and even 
sometimes put maps online of the dealer's room, 
with each vendor's position indicated. You can 
check it over for recognizable names... or, if 
time is too short, you might email the Exhibit 
Hall Director directly and see if he's aware of 
any cel vendors who will be present.

So, yeah. If you're doing the con thing, and 
you're undecided between two small cons, it's 
probably better if you make your decision based 
on a variety of criteria, as opposed to basing it 
on a single one... it takes ten minutes to flip 
through a table full of cel binders, and it can 
be very disappointing if there's nothing 
interesting to do after you've decided there's 
nothing interesting to buy! :o)

Good luck!