Well, if you're dead set on buying something on Monday... I'll agree with the others and go with a Glock 19. Reliable, concealable, and will work well in competition shooting.
I would advise against going with any sub compact if you plan on competition shooting. Sub compacts in general have poor ergonomics (mainly in the size of the grip) and the only way to fix that, as you noted, is the sleeve that goes on the bottom of the mag. That means you'll have to buy a sleeve for EVERY mag that you plan to use in competition which could be 5 or more, and when you start buying those in quantity, it really adds up. Mags are expensive enough without buying an extra $10 part for each one. Then, something you won't always hear, is that mag changes are more difficult with those extensions. The heel of your hand catches on the bottom of the empty mag and sometimes holds it in place. Not something you want happening when you're trying to do quick mag changes. You can change your hand position every time you want to swap mags, but that gets annoying and takes time that adds up if you're really counting.
Then with the sub compact, you're also loosing sight radius and barrel length. Both are negligible losses in overall performance, but every bit helps when you're talking about competition.
I would also advise that you spend more than one or two days picking out a gun. Stop and take some time and think about exactly what you want the gun to do, and then look at what requirements you have to have in the gun, and what you want but don't absolutely need. Things like size, caliber, trigger, capacity, weight, and mag price are all important factors to look at and will play into your decision. For example, I love the Sig DA/SA trigger, I think it is the smoothest DA I've found in an auto and the SA is very accurate, however the constant trigger pull on my G17 is good enough accuracy wise and in my opinion is a bit faster and doesn't require either the transition from DA to SA or to be decocked upon holstering. Because of these differences, I use a Sig 228 for CCW where I won't have to quickly holster and I like the better trigger, and use a G17 for my "go to war" rig with a Safariland holster where I like that I only have one trigger to get used to (as opposed to both DA and SA) and I don't have to decock it when doing transitions between pistol and rifle. What you plan to do with the gun and how you want the gun to do it is very important when choosing a gun you plan on using a lot.
I know I've gotten a little off topic for this thread, but to get back to the point, if you're absolutely getting one by Monday, I'd recommend a G19, if you have time to wait and find a deal, I'd probably look for a Sig 228/229/226. In your price range, it'd have to be used, but Sigs are very durable and a used sig won't be a problem if it was cared for properly. I'd also recommend you stick with 9mm as it is effective enough for defense and has low recoil and is cheaper than most other centerfires which will be good for practice and competition.
