Spot on take. I've subbed to both for years.
Video quality is markedly better with MLB EI.
I've had MLB.TV for the last couple of years after dropping MLB EI. You're correct, PQ is not as good on MLB.TV; don't think that there's any question about it.
However, I go with MLB.TV because it's significantly cheaper, and for me, the difference in PQ isn't enough to make me pay extra for MLB EI. And, I also like that I can get RSN, Comcrap Philly, Comcrap Houston, and now, TWC Dodger feeds. I will give D* credit that the last couple of years they have had Toronto, Philly and San Diego feeds, but I kind of got soured by the carriage battles for years and years before they were added.
If you are a consumer, you must weigh PQ vs price/feeds, and then make your decision. I don't think one can say there is a right answer/wrong answer about which service is better. I guess the beauty of the situation is that at least there are options re. television feeds for MLB fans, and that's a good thing.