I've worked with both, and I prefer combined. The first place where I had combined PTO, they also had a very generous accrual rate (like 8.1 hours per two weeks--with a full day counting as 7.5 hours--capping at 37x accrual rate and no one questioning usage), plus like 15 set corporate holidays, so I never felt like I didn't have enough.
Meanwhile, at my last job, where they were separate, I hit a situation where I ran out of sick days but couldn't use my bank of vacation days so had to take unpaid time. It was so stupid because the sick day accrual rate was like 4 hours per month.
At my current company, it's combined, with a good-ish accrual rate, but most people usually just opt to work from home when sick, so it's not bad.
I think accrual rate is key to whether the policy is good.