I agree with Ludens, in the case of MTW I´d start with vanilla. Although, maybe XL would be good for starting as well, since you gain way more income from the lands you hold and have to rely less on the long and difficult trade setup. But it´s also a matter of the faction you choose. I have so far only played with catholic factions, so I don´t know about the muslim or Orthodox ones, but Spain has proved so far a relatively easy one - your provinces are rich in trade goods and farm income, they have iron, you can wage a war against the Almohads without the pope screaming at you and you have easy access to both the Mediterrean and the northern sea regions, which eases trade buildup.
However, as a beginner do NOT, under no circumstances, try and start with the HRE in XL. It´s big, it´s weak and it´s surrounded by factions all too eager to grab your lands. You won´t be able to secure a single alliance at the beginning even if you play on Medium. I´ve managed to finish a XL HRE game - after five tries, and survival was by mere chance. It´s less extreme in vanilla, if I recall correctly, but it´s still a tough nut.
England is pretty easy in vanilla as well as the only non-English provices (Scotland and Wales) on the actual island of Britain are in rebel hands and can be bribed into your realm (Wales should be bribed, anyways - that gives you Longbows in Early, way before they regularly can be trained). In XL it´s a bit more mixed - you get more protection of your English provinces, due to the removed landbridge between Wessex and Flanders (a very rich and therefore tempting province, consider taking it as soon as you can afford to go to war with France) but that also cuts you off your continental holdings unless you can build ships.
And read both the Beginners Guide and the Unit Guide of Frogbeastegg. They´re more than helpful, even for those who have played RTW before trying their hands on MTW (they also contain a lot info that can be put to use in RTW, it´s just a matter of seeing where the similarities lie)
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