The big problem with the ZX series was that it wanted to be another Metroidvania clone which was a genre transition too far. Namely because one of the biggest tenets for the genre is having something to find for your exploration, even if it's just 5 extra missiles to add to the pile, and there really wasn't anything like that introduced into ZX. In either game there's a grand total of 4 Health ups, 4 Subtanks, 8 ability chips, 6 random miscellaneous items ranging from things like bread and cake to a classic series E & W-Tank to collect, and a bunch of useless data files and key items related to sidequests which would likely net you one of those ability chips, in-game currency, or more useless data files/key items.
ZXA was a bit better than that by making you go out and find 4 Biometal ups instead of making you play 'guess the second hitbox that contains your prize from beating the boss that gets worse when you hit said hitbox' (at least one of the Models shows you where to not strike but all the non ZX related models suck in this game) as well as a few extra equipables (but they were regulated to a monthly rotation thingy instead of having to go out and find them).
Beyond that though there's little else in regards to extras. ZX1 had a single secret area that hosts a boss rush and an extra Biometal (that's even more ridiculous than Model ZX) for a reward for clearing it (but this requires making use of your GBA slot and MMZeroes 3 & 4) and a NG+ option to refuse Model Z and keep Model X as your default Biometal. ZXA only had Model a (not to be confused with Model A which is the default model) as a NG+ extra but is a royal pain in the ass to get as you need to clear every medal challenge against every boss (including, among other things, fighting a boss with nothing but your Giga Crush attack which either sucks or sucks harder, based on your character's gender, and takes forever to refill or longer if you've collected any of the Biometal ups by that point).
As for difficulty ZX1 is the easier of the two due to the fact that your default Biometals are based upon X and Zero respectfully and are both quite powerful in their own rights and are not hamstrung by Biometal (re:weapon) energy, whereas the other Models, while decently powerful, are largely overshadowed by the aforementioned two in everything but their gimmick. Plus you can ignore the Boss weakpoint gimmick and have your Biometals repaired later. Meanwhile ZXA is the harder of the two because Model A sucks and you don't really get a better Biometal than it outside of Model ZX which comes later. Also Albert is an ass and is stupidly difficult no matter which form you take.
Also for those wondering if you're missing anything by not playing through the X/Zero/Legends series, you kinda are. The ZX games were pretty much setting themselves up to be the bridge that connects the rest of the time line to the Legends series (at this point in the franchise the line distinguishing between human and reploid no longer exists which fits with Legends 2 revelation that pure humans no longer existed and that all the people you meet are artificial lifeforms). Moreover there's a ton of callbacks to what went down during the X/Zero series (spoilers obviously)
Eurasia which gets colony dropped in X5 (and was supposedly the end of the Maverick Wars) was kept quarantined throughout the Elf Wars (which was supposed to be a nebulous series of events but ended up getting twisted around to make room for a later X game that'll never happen) winds up recuperating to the point where both human and reploid life was sustainable which is really handy for everyone after Neo Arcadia (which was the place where humans gathered up during the Elf Wars) got nuked in Zero 3 except for a rear end in a top hat from the Elf Wars (and quite possibly further back) named Dr. Weil who decides to take the scorched earth approach to Eurasia with an orbital laser during Zero 4 but Zero puts a stop to that by taking down the laser (but doesn't get off in time and dies for the third time). The ZX games takes place a ways into the future where a bunch of Mavericks are showing up again and people are using Biometals to become Megamen (in a retardedly anime sort of way) based upon the major Reploids of the past (namely the four guardians) except for Models A, X, and Z which are all copies based upon Model W which was likely created by Dr. Weil or one of his progenitors and everyone who's using them is all his progeny.
For those who don't want to see me :goonsay: all you need to know is that ZX1 is basically MMX1 but preceded by the events of the Zero series, succeeded by the Legends series, and containing a lot more stupid anime bullshit than either combined. All I remember of ZXA's plot was an obsession with booty, "NO! You are the Model Ws! And then they were the Model Ws.", and Albert being an asshole while Thomas was secretly an even bigger asshole and Model Z being up to something.
Also admittedly, this guy
was right in regards to this series since it was trying to be both 'kiddy merchandise shilling anime bullshit' and 'chapter in long running video game franchise trying to appeal to fans who are in their 20s more or less'. Had the third game came out it probably would've straightened itself out (or at least figured out whether it wanted to be a true Metroidvania or not), but like I said many time in this overly long post 'that's all irrelevant now'.