I own both. The S:Flo2 is one of the best sounding DAPs there are, and definitely the best price performance of all DAPs over $100. It was a definite improvement over my Cowon S9 and I imagine it would be the same for your iPod. However, the UI is absolutely atrocious, and aside from SQ & the fact that the back smells mellifluous - there is NOTHING good about it. Everything else reeks of low grade construction, terribly thought out design, unrefined UI and software, an atrocious screen, abysmal battery life - everything. else. is. bad.
I use the Mamba at home as both a DAC + amp (but never as a portable). It sounds great - even better than the S:Flo2, as expected, of course. The difference however, is more subtle - though more refined, with better decay, and better bass impact. Treble extension is also improved. Note that if you want to use this as a portable, you'll need a LOD, which adds a lot to the price as well as buy your own 9V battery for the Mamba. I've never tried the Mamba as a portable amp, but I imagine it would be similar to the S:Flo2, maybe marginally better. However, it is more expensive, a lot heavier, and far more cumbersome.
It really comes down to what you do more - want a nice sounding portable setup that is less hassle or you want the best of performance, but at the cost of a lot of effort. Note that buying the S:Flo2 may allow you to sell your iPod, which allows you to buy the Mamba with the money recuperated