As far as I know, aptx is the better profile, but iphones do not support it, so I guess you are using AAC.
APTX is a profile that supports FLAC / ALAC type streaming. AAC is a codec which means that the device in question supports being able to play AAC files (or read the codec). In otherwords, if the files are encoded with AAC, or MP3 (most modern BT devices support the MP3 codec as well as others), then you'll actually end up with a lossless signal of the lossy file as it ends up being the BT module that encodes the AAC data that is sent over the air (instead of the low-grade, SBC codec default). Others may be supported as well: MPEG-1, MPEG-2, MPEG-4 (normally used for video files).
Before APTX, there was no container (AAC, MP3, MPX, MPEGX, etc) or codecs that supported a lossless one-to-one correspondence over BT. That is why APTX was created. It supports converting that FLAC/lossless file to the APTX codec (no data lost doing this, it's like converting WAV to FLAC, or CD to FLAC).
That said, APTX isn't a better profile in contrast to AAC if you're using AAC files... If you're using MP3 files, your device will not use the extra processing/bandwidth/processor to convert the MP3 into an APTX compatible format, it'll send it as MP3. APTX is only used if you are streaming something lossless, otherwise, it's a waste of energy, power, and processor to use that codec.
Well the songs I'm listening too are mostly encoded AAC but yeah I've always heard that aptx is superior. Thanks!
You default to AAC if you're using AAC files... If you're using MP3, you default to MP3...
For those more interested in how these work: http://en.wikipedia.org/wiki/Bluetooth_profile#Advanced_Audio_Distribution_Profile_.28A2DP.29
Essentially, the low-grade SBC is a default that must be supported by the A2DP protocol. Other file-type support/codec support is optional, and if its there, it'll be used.