Hey had to make the same decision between the ONkyo 715 and Solo Mini. So I went to my local Hi Fi retailer in Bristol. Done a head to head sound audition, they used the Arcam Muso speakers. The sound quality was definately better on the Solo Mini, particularly the base. I was not that impressed with the 715, after reading so many good things about it on the net! So I bought the Solo Mini with the rDock.
The Onkyo is now ageing, the Solo Mini is new and looks better, really understated, no writing such as DAB, Mp3 etc. Slide CD is more desirable.
The USB is a good thing to have (none on 715) and works really well, with pen drives. And as you mention you have Mp3's, its important.
The rDock also works really well with the Solo Mini.
DAB is also very good on the Mini. But you need to use an old skool steel extending arial, which ruins looks a bit.
My disapointment with the Mini is that, its power is not enough, halfway volume just for regular listening!!
When using Cd with MP3 files, its noisy when changing tracks and can be slow. I do not recommend listening to Mp3 in this way. Instead I use my rDock or the USB pen drive. Both work fine.
Cd/ rDock does not retain position, when changing between sources. My CD/ iPod in my BMW does when I turn off or change sources.
Overall I am fairly happy with the Arcam Solo Mini and would recommend it.
Hope that was helpful.
Price £585. rDock £110