on adsl different providers can have different stability this is because LLU providers have their own dslams and as such may run not only with different chipsets but also a different configuration can lead to better or worse stability. My personal experience is if you on adsl look for a provider who has no DLM for better stability, DLM tends to make things worse.
On FTTC, all providers in the uk use a openreach cabinet dslam and as such stability only depends on the DLM profile, with most isp's I expect on the speed profile but at least two I am aware of use the standard profile, there is also a stable profile. The profile controls the level of instability required for DLM to intervene.
I woudl think for the vast majority of people FTTC will be more stable than adsl for 2 prime reasons.
VDSL is newer tech than ADSL so that alone should lead to stability improvements with the protocol been more modern. Second VDSL removes the E side copper out of the equation so only the D side can cause problems. Generally this should mean a shorter line for broadband purposes and more stability/speed. In my case the E side made adsl extremely unstable without SRA, and so on FTTC those issues are a thing of the past.