First off, any contact martial art (and possibly sport) in which you hit, get hit, and focus on conditioning will leave you off better than before. But a few counter-points on why I don't believe Muay Thai is superior over Boxing for a street fight, especially for those leaning towards it for a self-defense. I'm not trying to get into a big "versus" thing here, just food for thought on an intellegent discussion like this one. Disclaimer again: Again not saying one is absolutely better, just some counter-points- as we all know it's the person that matters, right?
First, elbows and knees aren't illegal necessarily because of effectiveness but for purity of the sports (usually a stand-up striking event), because once in that close in, the best option is often to begin grappling to shut down such strikes. So there's no point in reducing a striking match to grappling or letting athletes get nailed by blows they otherwise could avoid by grappling (sport Muay Thai has rules that minimize both problems so knees/elbows are used). Not that I wouldn't use a knee or elbow given the opportunity, but most boxers know how to throw an elbow (just miss the hook slightly) and many will see a knee opportunity in a clinch (a common "real fight" situation that the boxer will be more familiar with than your average drunk joe).
Second, intensity is not necessarily a good thing for a self-defense student. Muay Thai is a bit of a fad so they'll be many "fake" MT schools, but the "real" stuff and the conditioning required might be too much for anyone with only a day or two a week to spend. So most Muay Thai schools train students loaded with safety equipment and RESTRICT the use of elbows and knees. The brutality of the sport in Thailand says little for someone trying to learn it here and might simply mean the lives of the combatants are worth less than the entertainment of the crowd. Boxing is straight-forward, practically a combative. You'll know real quick if you're training at a hard boxing gym or an aerobic class... with MT, the distinction isn't always immediately clear, especially with inflated class prices (hey, boxing's often FREE!)- people get into the mentality, "I'm paying a ton of money for this so it MUST be good!"
Third, the proof is in the pudding. When Boxers and Muay Thai fighters meet, the Boxers most often wins. More Boxers successfully cross into Muay Thai than vice versa. On the street? Yes, a weakness of boxing may be the forward foot (though shouldn't take much to train on defending that), but Muay Thai suffers from an open guard, square stance, and low aggression (IMO). Watch some MT matches and you'll regularly see these guys popped in the jaw, the only reason they don't fall is because their square stance lacks power (arm punching/punching from the shoulder vs hips- like a hockey fight)... because of the energy and the "deadliness" involved they're also rather unaggressive if you ask me, with a heavy focus on defense. Could just be the lack of combos though.
In Boxing Heavyweights look for the quick KO and lighter classes go for points with furious flurry of combos (which really work effectively in most "real" fights provided you flurry first). When dealing with inexperienced fighters, low attacks aren't really common at all, though you can expect attempts at kicks to the shins/groin, "judo" take-down kick in a grapple, or tackle/shoot for the legs. For you're kneecap kick, the boxer just moves his feet, or lifts his leg slightly... then KOs the sucker while he's off balance (if the punch doesn't do it, hitting the floor will).
Just my opinion, but I'd get into one of those real boxing gyms and use the money saved towards BJJ or save even more money and wrestle instead of BJJ ('sides nearly all wrestlers are into MMAs or xtrain) then spend the money on knives, guns, cars, and training in their use!