For the professional live DJ, you can't beat Traktor DJ Studio by Native Instruments. Automatic beat-matching and tempo synchronization, plus scratching, filtering, kill switches, track management, multiple outputs (for preview & cueing) and more. Everything the live DJ needs.
For the studio DJ, MegaSeg looks like the better choice. I've never used it, but it looks like they are geared more towards automation and time-management...great for radio, but not what the average club/party DJ is after.
For real-time pro-level control of the mix, Traktor rocks. It's what I use.
p.s. Any software that supports cueing with multiple outputs (MegaSeg, Traktor, or whatever) is still going to require that you have at least two stereo outputs on your system. Since none of Apple's current line of laptops have more than one out, that means you need an accessory. The Griffin iMic is sufficient, but you can also spring for something like the GigaPort AG, which adds an additional 8 channels of output. That's 4 extra stereo outs, or enough to support DTS 7.1 surround sound, if you want to connect your home theater. There are also several options out there that also give you multiple inputs and/or MIDI, which is handy if you're not only a DJ but want to produce your own music.
Anyway, Native Instruments even offers a specially-priced bundle of the GigaPort and Traktor, which gets you everything you need.
Traktor DJ Studio 2.0
M-Audio Firewire 410
(one of many high-end input/output sound interfaces...4 inputs, 10 outputs, 2 headphone ports, and a lot of other goodies for about $400)