Here is my advice.
What is play like at your table. If you like telling a story with your players, are happy agreeing what is credible or not, or how hard something is among yourselves, and want to roll to inject a little bit of uncertainty in the proceedings, but don't want to step out of the fiction into mechanics too much - opt for Heroquest. If you like to emulate what it would be like to live in Glorantha, want arbitration on what is possible, and like a tactical combat system - opt for Runequest.
There are other trade-offs, prep-time, what you can cover in a session of play etc., but for my part it mostly amounts to a stylistic difference, so you have to figure out what you want to play.
For my part, I like the fast-paced play of Heroquest, the feeling that I don't interrupt the role-playing for roll-playing that often, but you may prefer the feeling that you are living in the world with a realistic portrayal of what that means