IE7 - In place upgrade vs. slipstreaming

[TechMonkey]

(Wasn't sure whether to put this in unattended or here, but it's not strictly an unattended question...)

I'm wondering what are the actual, real differences between having IE7 installed on top of IE6, and having it slipstreamed into the source?

All things being equal, I know having stuff slipstreamed is "cleaner", and I'd prefer it, but are there any functional differences? Just a bit of extra disk space? If I install w/ the "don't backup files" switch, is it the same or...?

Background - I'm creating a new build image - before it was an SP2 install I had slipstreamed IE7 (along w/ several other items). W/ a fresh SP3 install source, I'm having trouble slipstreaming IE7 in, and don't want that to hold me up any longer if it's 99% the same...