In terms of quality of image and sound, blu-ray is far superior. DVDs can be a mixed bag, depending on whether they are modern productions with good transfers and good sound tracks, or old products with grainy image and lousy sound. But in general, you're better off in terms of quality of image and sound when playing a decent DVD or any blu-ray rather than watching streaming video on the Internet, in spite of the fact that MetPlayer does have decent video and sound quality if you have a fast enough connection.
In terms of choice, there aren't many operas on blu-ray, but there are hundreds on DVD, while MetPlayer has a limited selection - first of all, thanks to only Metropolitan Opera productions being shown, and second, thanks to many of their productions not having been included in their selection, although they usually do add one or two every month or two.
In terms of price, of course MetPlayer has a very attractive price, since for half of the price of a DVD and one third of the price of a blu-ray you can watch as many operas as you're capable of watching in a given month.
However, there is also something to be said about physically owning an opera. Some people are collectors and won't be satisfied by merely subscribing to a website, but will want to know that they have a rack full of opera DVDs or blu-rays, and if they have friends who love operas, they can lend them, take them to opera-watching soirees, etc. At times, watching an opera on MetPlayer just makes me more likely to buy it as well, so the savings may be misleading.
Depending on where you live, you can also look into rental services like Netflix - they do have some 300 or more opera DVDs that you can rent on unlimited bases paying a fee of about 9 dollars per month, with also the perk of being able to stream 35,000 movies (no operas) directly to your TV (or your computer, anywhere). Public libraries are another good source.
There are a few free sources of opera streaming, and these have been covered by one of our threads; browse the forum and you'll find it.
Finally, you should also consider the market of used DVDs. I've been lucky every time I bought used DVDs, and haven't found a single one to be defective yet, but paid a fraction of the original price.