It’s one of those “if it’s not broke, don’t fix it” issues. Can you ride comfortably with what you have? If so and you’re curious, find a friend and borrow a pair of “fancy” ones before buying.
I have bad knees. The MDC stirrups with the 90 degree rotated top were a life saver for me. I went from extreme pain, knees giving out when I hopped off to after 30 minutes to … nothing. Knees are fine after hours now and I’m not longer fighting to keep my toes straight.
Now that said, I have the swivel ones where you can pick flat, 45 and 90 degree angles. These are no better for my knees than the set 90 degree ones and they cost a lot more. I could have made do with the set ones but these ones store better and they were a birthday present and I love them.
I have tried the flex stirrups, didn’t like them. They seemed to encourage me to reach with my toes instead of sinking into my heels. I’m sure I could have gotten used to them but solid works fine.