A few years ago, I migrated from a 568v2 to an 861v6 in an analog-amp system. Based on that comparison, I'd characterize the 568 doing DAC/preamp duty as having a really sweet, friendly sound which however provided less of that subtle raise-the-hairs-on-your-arms detail than the 861. I preferred the 861 overall, but still slightly missed the friendlier aspects of the 568 until the 861v8 update came along.
So... perhaps this will provide some hints about what a 568 might offer you in your system, although everyone's mileage may vary.
The 568 also has the feature that it can drive a balanced power amp properly, since it has balanced outputs for the front three channels.
The Prime is a more-modern design throughout. If you want something compact with a volume control, are satisfied with single-ended preamp outs, and want a headphone amplifier (but don't expect to use balanced headphones), the Prime is a fine choice and sounds good. It's a desktop system, and as such doesn't have a remote control. (Or have I just lost mine?)
Note that if you're looking for a straightforward D-to-A converter, while the market is extremely crowded, I suggest you look at the offerings from
Schiit, especially their newish "multibit" models. The least expensive of those is the
Bifrost Multibit, whose sound I've found I prefer to the Prime's built-in delta-sigma DAC; this may of course change for some recordings once the Prime's MQA firmware update appears.
I've found that so far every moderate-priced (which is to say, under 2k USD or so, which is as far up the ladder as I've gone) USB DAC I've tried has benefited from having the Schiit
Wyrd (a USB power supply substitute and data reclocker) in front of it. To get back to Meridian-land, I prefer the sound of the Prime's DAC and the Director when driven by the Wyrd. The Director, for instance, sounded a little bit lightweight driven directly from a Mac Mini's USB port, but gained body and listening ease with the Wyrd in place.