M1 and M2 are physically the same. the M2 was marketed differently because they bundled the sligbox app with it for free, where the M1 at the time still required the purchase of a $14.99 app.There isn't a good substitute for a Slingbox because it is the best at what it does.
While SlingMedia is no longer maufacturing Slingbox's, they still sell the remaining inventory (Slingbix site, Amazon, etc.) and will support latest gen of Slingboxes. The Slinbox latest gen for sale with continued support are th 500 and the M2. 500 uses on-screen setup and has HDMI, Component, and Composite, with up to TWO different devices connected and swtching between devices to stream. The M2 has Component and Composite for one device.
Slingbox will stream OTA via HDMI because OTA is not flagged at all. However, if you wish to stream flagged content, then it must be via Component connection.
I have two 500's and they outperform my sad TiVo stream with near perfect reliabilty and better PQ, especially with slow internet speeds. Even DishAnywhere can be flakey (same Sling technology--Dish owns Sling Media--but far more complex app and services) at times (although not nearly as utterly useless as my TiVo Stream), but my two Slingboxes have been rocksolid by comparison.
Don't bother buying older legacy Slingboxes at fleabay, etc. They are no longer supported. 500 and M2 and possibly M1 are supported. I should state that SlingMedia has announced only no longer manufacturing of Slingboxes, but they also announed that latest gen Slingboxes will continue to work for the future.
Of course, with each payTV provider offering its own app for viewing content and with Netflix, Hulu, Amazon et al. the need for a Slingbox is not what it once was. Get ur Slingboxes while you can.
but now, the app is free for anyone, so the difference is irrelevant.
I actually just got an M1 box off of ebay for like $50 shipped. The m2 seems to go for about $75. no reason to not get one off of ebay IMO.
to the OP, i also had a solo that was starting to die, and i decided to get a replacement for the following reasons:
1) the solo was not HD. and any time the bandwidth was not 2000kbps or faster, the resolution was really annoyingly bad. with the M1, i get pretty good resolution at much lower bandwidth
2) i have an amazon firestick that has the slingbox app on it. The M1 is supported to be watched with it, the solo is not. So that just makes it even easier when i'm traveling to watch via the firestick. Before, i had to stream it from my tablet, and if i wanted to watch it on a TV, hook up the tablet via an on the go cable that i plugged into the TV. really just a cumbersome solution.
it may be worth it to just spend around $50 for a new to you M1.