For those interested, a relatively recent ( a year ago) investigation of the hydraulics of SJM vs. On-X was published in the Journal of Thoracic and Cardiovascular Surgery Vol 159 #1 by Hatoum, Maureira, and Dasi. "A turbulence in vitro assessment of On-X and St Jude Medical prostheses" (I'd paste a link directly by my version prohibits distribution!). I know most normal people aren't interested in hydraulics . . . but I am! Anyway: here's the conclusion .. .
Conclusions: This study shows that despite the design differences that characterize the On-X valve, the hemodynamic and turbulence parameters were not necessarily improved compared with SJM. (J Thorac Cardiovasc Surg 2020;159:88-97)
Another relevant summary sentence: This study shows that despite the design differences between the 2 valves, hemodynamic and turbulence parameters are close.
The On-X valve showed some small pressure drop/area advantages, but it's leaflets showed some minor oscillation giving SJM some minor turbulence advantages.
As others have said, if you already have your valve or are choosing one, the differences hemodynamically are not very relevant. If you do not have a new valve (yet) or are having difficulties, the much larger impact comes from the valve area itself. Likely a non-issue unless you are younger, very active, etc. and can truly benefit from a larger valve. In those cases, I do think that a discussion of valve area with the surgeon in advance can be beneficial. At least it was for me.
If the bioprosthetic valves would just last forever . . . . . . . .