Willie - my 2 cents, though I’m not certain about some of these (where indicated).
1. which service offers the highest resolution for $10/month?
As from gdnrbob & jond Spotify Premium is ~$10/mo and Tidal HiFi (their best) is $20/mo. SQ from Tidal is definitely better than Spotify. BUT for me, the Tidal trial version using a Macbook Air via USB to external DAC did NOT clearly show this. The difference at that time between Tidal & Spotify was NOT night and day. However, once I got the Aries Mini that came with a 1 year Tidal subscription, the Tidal SQ advantage was clear. Spotify seems a little closed in, just not as much air or "sparkle", less lively compared to Tidal. You can "hear" the compression in Spotify though if you aren’t A/B-ing, Spotify is still very listenable. Tidal is VERY close to ripped CDs though I still prefer the latter.
2. which service has the most intuitive interface
Personally, I find Spotify easier to use. Prefer the way it is organized. Tidal just seems more cluttered. BUT it’s not a deal breaker and I do have a lot more time and experience using Spotify. The Browse/Discover function on Spotify has turned me on to quite a bit of new music.
3. which service can be accessed via a computer interface - makes downloading into my library easier
With both Tidal & Spotify there are downloadable "apps" that can run on PC, Laptop, or smart phone. With both you can "save" full albums. Tidal lists the ones saved under Albums or My Music (accessed in the left sidebar). Spotify shows my saved albums under "Your Library" again found in the left sidebar.
Spotify definitely limits the number of albums you can save to your library. I don’t know how that limit is determined. I thought it was just based on hitting some max number of albums. Don’t know for sure though. I’ve run up against this limit several times. I just delete ones I've not been listening to very much. Don’t have the same user history with Tidal, but so far I’ve not encountered a limit to the number of albums saved on it.
4. which service can you download music to play offline
- is there a limit of the number of offline songs
- is the limit a "total number of tracks" or "total" per period of time (quarterly, annually?)
- can the songs be stored on or moved to a NAS drive
So far as I know both let you download for offline playback. I don’t know the limit for downloaded albums. With Spotify, these saved albums do seem to occupy space on your hard drive (I can see 'em in Finder)....or wherever you have the Spotify "app" saved, I think. But one of the virtues of Ogg Vorbis compression is they take up less space. I am not as sure about how or where Tidal albums are stored. Given they are uncompressed they’d take up a good deal of space on a local hard drive. I’m guessing they reside on a remote server you access when connecting to Tidal BUT THAT IS A COMPLETE GUESS on my part.
Just because an album is saved to your Spotify Library or Tidal Favorites does not make it automatically available when off-line. I’m more familiar with downloading for off-line playback using Spotify. You have to be logging into Spotify at some minimum frequency (1x/mo, I think) to keep your downloaded albums available. Not sure about a limit on the number of these you can have. Almost completely certain you can download with Tidal for off-line playback but I haven’t done it so can’t provide details.
I don’t know of a legal means for saving Spotify or Tidal music files in a completely independent, off-line mode. If you had Spotify or Tidal programs running on an external drive, saved albums or downloaded files would be on that drive too - at least for Spotify; not sure about Tidal. As I said, not sure where saved Tidal album files get stored.
5. which streaming service crashes the least
I have NOT had "crash" problems with either service. However, I still get occasional interruptions due to buffering with Tidal. Doesn’t happen much but seems to run in streaks. Distance from wireless router does seem to influence this but this is not the complete explanation. Spotify has been pretty flawless in operation and I’ve been a subscriber for a couple years now.
I use the Browse/Discover function on Spotify. This is a great feature. Tidal has something similar, I believe. If I find something I like, I’ll look for it on Tidal to enjoy the better sound quality. However, the Spotify library continues to be much more complete than that of Tidal...especially if you like to get off the beaten path in pursuit of more obscure titles. In my O-pinion that is one significant advantage Spotify does have over Tidal.
If you want to get started, $10 per month for Spotify ain’t much. If you like it, try Tidal next and if you like the Tidal SQ advantage vs the more limited library, dump Spotify and go with Tidal. Ain’t rocket science, dude :-) Good luck!