Choice depends on if you think MTB riding is something you will really get into. Mossy gave me some great advice when I was first looking.
Its a balance between what you need and what would be nice to have. Hardtail 29er for sure and spend as much as you can afford. As you've pointed out, the Marlin is a good basic bike but you will be disappointed with it if you do more regular ridding. The X-Caliber 8 is more that you need now but will be a much better deal overall if you get into the sport.
Jump on to bike exchange https://www.bikeexchange.com.au/s/hardtail-mountain-bikes?option-value-ids=4005-4015 plenty of good deals on last years models etc. and don't limit yourself to any particular brand as all the big names produce pretty good stuff.
There are plenty of Facebook MTB buy sway sell pages as well were you can get really good deals. I've bought 2 bikes this way.
Post up any options you are looking at if you want advice.