I currently use both of these (for my SIPP) and I think I’ve used or tried almost all UK brokers at some time. Most of these experiences were 15+ years ago, and the experience was horrible for anything except HL and II
secure messages are good enough for handling correspondence etc. but having the option to pick up the phone and talk to someone in next 2-3 minutes is fantastic when you need it. I just did this, with II last Friday 3:55pm and it was a lifesaver.
also HL fees (when other brokers put HL in comparison lists) seems infinite, but the fees do cap annually so it is roughly same cost as II (which has a fixed cost regardless of size)
My only problem with both HL and II is, they have “criminal” FX conversion fees. The upside is they support holding money in several difference currencies so you rarely need to do conversion. BUT if we are exclusively talking about ISAs, for some weird reason HMRC does not let you hold FX in an ISA so you’ll have to keep converting.
How “free” T212 is a debatable topic, that cannot happen in this forum 