I'm autistic and have subclinical BPD and yeah, the meltdowns caused by each are different. When it's just sensory overload, I'm often not feeling any negative emotions, just feeling overwhelmed and tired and like it's all too much, and I often lose skills. (I don't have a need for routine, but I do get overloaded more easily in unfamiliar situations.) When it's the BPD tendencies, it's pretty much pure emotion, and I usually don't lose skills, but I feel like whoever upset me is my enemy and is determined to make me miserable. It almost feels like they're sending pulses of negative emotion at me. It's also worse the closer I am to the person, because if a person I'm not close to upsets me, I just leave, but if I need them emotionally then I can't leave.
The management is very different too. With autistic meltdowns/shutdowns, I've found just going somewhere quiet and doing something calming makes it better, whereas with BPD-meltdowns I need to convince myself that my loved ones care about me, often by screaming at them until they give me a hug. (Yes, I know it's not the best strategy, but I can't stop myself from doing it anyway.) And over the long term, I've mostly eliminated autistic meltdowns by just avoiding overloading situations or leaving as soon as I start getting overloaded. But there is no way I could avoid BPD triggers because they're tied to a basic psychological need (love). And even when I'm alone, I'll start getting depressed if I'm alone too long. So instead, I'm trying to treat it by learning alternate coping strategies using a treatment called DBT.