Originally Posted by
subgrade
This. Looking at Trek's website, Marline 6 is the lowest spec which has freehub as opposed to freewheel on the 5. Freewheels are limited to 7 speeds, among other disadvantages, and are becoming obsolete in general. New 7-speed parts by now are limited to the low-end only. That means if you eventually want to upgrade the drivetrain, which is very likely, you will have to change the hub too (or probably the whole rear wheel while you're at that).
I don't think this is true at all, the Marlin 5 has a normal freehub and 7-speed cassette. Heck, I have a freehub/7-speed cassette on my 30-year-old Trek Antelope 840! It's turned into my daily commuter, and I've had no problems buying replacement cassettes every year or two. I suspect that will be true into the indefinite future.
That being said, I have a 2016 Trek Marlin 7, which I find perfect for tooling around on backwoods trails. If you're looking to purchase a new entry-level mountain bike I think the Marlin series is a great choice at whatever price-point you're comfortable with.