well, I own both IEM's, but not the sflow. I use an iphone 4 and a sony X, both are good. without an amp, I find the TF10's to be a good pair with the iphone, the TF10's tend to be a bit recessed in the mids, but are very easy to drive, and thus seem to even out the iphones warm/live sound. The UM3X are very good also, but being a more neutral IEM, they tend to sound a bit warmer and mid heavy through the iphone, not that they are bad, just different. Now, with an amp (mine is a mini3) the UM3X is the clear winner, while the highs are still not as sparkly as the TF10's, the sound is much fuller and the low end is very, very good. separation is better, and the high end tends to stand out more. So I find that if i dont feel like luging the amp around, i use the TF10's, but if im going to be listening for awhile the mini3&UM3X are the way to go.
Now with the X it is a bit different, I find that the X does have a different sound that the iphone, but to me thay are quite similar when no EQ is used. but the TF10's can get a bit shrill on the high end on some tracks, but other than that it is a good pairing. The UM3X are better overall i feel with the X, and the X has a much better amp in itm ( i think) cause the addition af an amp has less overall effect on the SQ.
The Sflow, from what i have read, has a pretty good amp in it, but the line out is begging for a good amp, so my nod would lean twards the UM3X's. but for techno, you might find the TF10's to be more to your liking. The UM3X's are more of a balanced sound, so they are not a bass heavy IEM, to me the lows are just right, not overpowering, but very well controlled ( especially with an amp ) and in the right amount. The TF10's seem to have a bit more of a low end bump, and may give you more of that "thub thub thub" sound that is prevalant in techno and hip hop. They are both good choises, and if you can, i would recomend getting both, and keep the one you like best. if you are carefull and keep the origional packaging, you should have no trouble selling the unwanted pair, and only being out a little $$$ for your testing. That is how I got my UM3X's, another member actually bought 4 pairs if IEM's, and sold the ones he didnt want to keep.
The TF10's are by far the cheepest, i got mine for ~$190 on amazon. but I have never seen the westones for less than ~$350-$360.