Digital

I think your insight into the two platforms is pretty spot on IME. I've played on both and have enjoyed them both, even preferring MODO's tabletop imitation, and I think your answer to a few questions would steer you in either direction:

• Do you want to play quick games or play anywhere? If so, Arena is definitely the way to go. Being able to play at any hour, for as little as 5 minutes for a Bo1 game, on Arena is really my only option currently with 2 small kids. If you enjoy and have time for true Bo3 matches at a computer, however, MODO would be better as I think the skill level is a bit higher and you get access to all formats down the line.

• Do you care about getting your money back out at some point? Arena is essentially a video game and once you buy packs or use wildcards, that's it. You cannot trade cards and you cannot exchange them. If this doesn't bother you, or you intend to play for free for a while as you build up a collection, Arena is great. MODO has a higher upfront cost (depending on format) and does not have a free-to-play option, but you can always sell your collection or trade your cards with others.

• Do you want to play custom formats, or older formats, like Pauper, Pioneer, Modern, Legacy, or Vintage? MODO would be your only option then and I would say even Pioneer is a ways away on Arena.

• Do you mind paying for every match you play? For the most part, MODO is built around "leagues" where you pay $10 for 5 matches. You get actual rewards based on your wins. There's a free play area for constructed though, I've never done that. Arena has paid entry events (with either gold you get for free in the game, or gems you buy for cash) but their ranked queue is completely free to play and you can essentially play a deck forever for free in there.

You're right that Arena is certainly WOTC's main digital priority and has the most upside in the long term. MODO has been around for a long time though, has a stable economy, and there's no indication they're getting rid of it.

tl;dr: I just don't have the time in my life to queue up for MODO leagues and if I play digitally it's just Arena purely because of how quick and easy games are, but if I had the time and the 2 little near constant interruptions stopped a bit (😜) I would invest in MODO to play the older formats I like and because I support that style of economy much more than Arena's which often feels predatory.