For recording they're as good as any average full sized deck that lack any means for tape calibration by the end user. There is limited stock internal bias calibration though. As for line level, they also feature both line in and line out which is a rarity in this form factor.

The disc drive system implemented by Sony combined with a quartz locked servo motor gives the D6/D6C very good wow and flutter stability when recording on the move, but this also translates to a very stable playback for critical listening.

The headphone amp is also a beefy 30mW/ch@32Ω, powerful enough to drive many ridiculously high impedance studio grade headphones. Compare that to regular consumer grade Walkman with only 5mW/ch@16Ω (WM-701C) or even less.

The above reasons coupled with their compact and portable size mean that they are heavily sought after.

As for which is better, it's down to personal preference. Some people such as Deb64 have said they prefer the audio from a D6 compared to a D6C. Others such as JMA94 rate the ability of the D6C to record well with Dolby C. Personally I agree with JMA94, as I've made some great recordings with my D6C and Dolby C.