I think DR7 will make better recordings, than MZ-R50, because MZ-R50 uses Atrac 4.5, which is pretty old now. So, I am going to use MZ-R909 for recording anyway, because it has Atrac Type R, which is better, than any other Atrac on the market. I am really suspicious to Sharp Atrac, since the previous versions of it were far behind Sony's ones (even Atrac 4.5 was much better than Sharp 6), but I believe they made a good progress since then. I didn't have a change to compare DR7 and 909, but I would still prefer Sony's Atrac, since Atrac R work perfectly for me and I don't think it can be any better. As for Sharp, don't know if they managed to get the same quality, I will doubt it.
But anyway, my advice would be to use a home recorder for making recordings and a portable player (either DR8 or E10) to listen on a go, if you are connecting your headphones directly to a player. The headphones, which you can use directly out of portable are Etys 4P (my favorite), Beyer 250-80, Ultrasone HFI-650 or Sony V6. However, if you have a portable amp, then I would go with MZ-R50 for a portable player + amp + same headphones or some other ones with higher load, like Beyer 750-250 or 250-250 or Etys 4s. All the headphones I am talking about are closed, because we talk about use on a go. Many would recommend Sony Eggos, but they are out of the league, I mentioned earlier, however are really portable and comfortable.
The reason of using a home recorder is that it has a better transport, much better DAC converters, less vibration and less power fluctuations dependant. Of course DAC depends on the model you use, but any middle-end Sony recorder will blow any portable player away in the recording quality. Use best of the two worlds! And yes, even MZ-R50 is backward compatible with Atrac R (it only matters, when you encode, for decoding you are using standard Atrac), thus will give you much better quality with good Atrac R recording.
So, record in Atrac R on Sony high-end MD recorder connected to high-end CD player through coax digital using perfectly mastered CD, listen to it on MZ-R50 connected to a portable Meta42 + Etys 4s, Ultrasone HFI-650 or Beyer 770-250 or 250-250 (depends on the music you listen to) and you will be in nirvana on a go. Just be careful or you might be hit by a car accidentally