A friend of mine is in the same position right now and is in the market for a new box for the same price point as the H96 Max S905X2.
So my starting point for consideration, taking into consideration general spec and price point is that I would immediately say forget the S905X2 and focus on S905X3 as price differential is very small and so best to go with the later SOC, which may also have longer term support.
The key differences between S905X2 and X3 boxes is that X3 boxes tend to come with 1 USB 3.0 port as standard, the X3 is faster and is also more power efficient.
But first a little bad news. Just about every cheap Android TV box (and rather too many expensive ones too) comes with crappy firmware, most of which may see an update or two that will fix little, leaving it in the hands of independent devs to produce custom firmwares to make them more practically usable and stable. Which boxes get such support can often depend on what is popular, so if Android is a really key priority for stability, performance and reliability, do some research on sites such as Freaktab, xdadevelopers and others.
For the purposes of media usage, then of course, CoreELEC is so much better than Android and has way better support by a country mile.
Most boxes will come with WiFi and Bluetooth and so I would say that if WiFi is important then go for a box with dual band WiFi and even better, supporting the AC standard.
Bluetooth functionality is fairly standard but as each manufacturer will use differing WiFi/Bluetoooth chipsets, do some research on here to make sure that the tyoe you may choose is supported by CE. Pretty much most WiWi are but Bluetooth is not quite as universal, although it is possible and cheap to add a generic USB Bluetooth dongle to provide support, albeit at the expense of using a USB port.
The cheaper the box, the more possibility of having 100Mbit Ethernet and whilst adequate for some needs, I would personally recommend going for something with 1000Mbit (Gigabit) as not only will it be faster for streaming but also handles higher bitrate videos without annoying buffering, especially via the internet (noting that slow internet connection means potentially slow streaming and buffering anyway).Some boxes with the same name will have differing specs in terms of Ethernet (and sometimes WiFi) dependant upon specs, so watch out for that as it can make a cheap purchase suddenly seem like a bad one.
Most X3’s will come with 1 USB 2.0 and 1 USB 3.0 and a select few will come with an extra USB 2.0, which might be handy if you need more ports for perhaps TV tuners and other things.
There is only one X3 that I am aware of that also has a 2.5" HDD/SDD bay.
Then there is of course the user experience and reviews, which it always pays to research on forums and places like Youtube. The time will be worth spending to ensure that you get what you want.
There are some fake boxes out there that look like the originals but have inferior and even different hardware under the hood. Fleabay is a particular place to find them, so always purchase with insurance just in case.
RAM and Storage will be dependent upon what you yourself need but with USB ports, micros SD card slot, you do get immediate storage expansion options. The more RAM the better but 2Gb should be the minimum to start with and perfectly fine for CE use but Android will use more RAM in general and if gaming is your thing, I’d say go for 4Gb.
For my friend, based on them not wanting to spend more than £45 I have currently recommended the X96 Air, with one proviso.
They can get the 4Gb/64Gb model, with Gigabit Ethernet, 3 x USB ports (2 x 2.0, 1 x 3.0), dual band WiFi, Bluetooth.
Reviews are good and user experience seems quite positive.
But the proviso is not one that is entirely uncommon to TV boxes, cooling/heat dissipation.
The manufacturers of the X96 Air failed to provide any vents for heat dissipation,which does mean a hotter box, although only seems to be an issue under high load/stress.
So I shall be taking the case apart and drilling some holes/vents into the plastic to reduce overheating, which may not be something that you want to do.
If your budget were to stretch further then you might even consider an SBC (single board computer) such as the Odroid C4, although that is an entirely different discussion.
As far as dual boot from eMMC is concerned, which has pretty much been covered by others, you did the right thing to ask if a particular box was capable and I don’t know if the X96 Air is, although there are a couple of discussion thread about it on here, so you may well find the answer there.
Good luck OP and be prepared for some ups and downs along the way but it’s all worth it for the most part.