Hi Nathan, I have grappled with this question as well and many people I've spoken with/podcasts/books have different opinions on what's best. I think it's all based on your risk tolerance and timeline.
I think it depends how much you are looking to build up to put into your next property and how quickly you can do that. If you're looking to make, say, a 20% downpayment max then it may not take too long to save up the cash (depending on your purchase price of course) in which case a money market account may be the best move.
However these days with talks of rising inflation keeping cash could also become a risk.
Here's what I do - and I am by no means an expert - I purchased my first property in November 2020 and am actively saving for my next. Initially I started putting every bit I've saved into an S&P Index Fund and various stocks. Now I am gradually shifting into cash and hope to have about 50/50 between cash and investments when I start actively looking again. Ideally my plan would be to use all of the cash and some of the investments to purchase the property.
I hope this helps - I always love hearing how others approach the issue.