Both are very viable options and deliver a similar look. Dye Sub’s popularity has exploded because it offers that look at a more affordable price and as mentioned is light and physically quite durable and can be displayed without a frame. While I still prefer conventionally well framed work (I believe a frame isolates the composition from the room and helps the art), framing is expensive so anything that offers a frameless choice is popular.
As mentioned face mounted inkjet offers several advantages, many times not an issue but in some cases a definite problem. Inkjet prints can use a denser dither (2880x1440 for Epson printers, 2400x1200 for Canon and HP) vs 720x720 or at best 1440x720 for dye sub metal. I’ve printed a few metals that just can’t hold the delicate colors in some soft gentle tones. To me facemounted acrylic has more depth, especially if an anti reflection acrylic is used such as TruVue’s TruLife (which also has a scratch resistant coating so it can be cleaned like glass). You can’t do anything about the reflections on metal prints. Of course this drives the cost up quite a bit. We have recently started offering face mount on 1/16fth acrylic backed by thin black sintra at the same price as dye sub aluminum, and quite a few prefer the acrylic.
As far as longevity, there are two factors. First is good acrylic such as truLife has UV protection in it, and some of the better adhesive’s also have UV protection. You can get chromaluxe EXT with some UV protection but the results seem to indicate it helps but isn’t as good. As far as the inks themselves, dye sub is a dye ink, not a pigment, and while it’s sort of encapsulated in the coating on the aluminum print, it’s not as encapsulated as in pigment inks. I’ve heard Wilhelm is testing chromaluxe but haven’t ever seen any results, but Bay Photo shows results of an extensive test which is on their site, and while they make it look good, the test only compared metal to chemical photo papers which we all know has pretty poor longevity. Added to that if you look at the fade charts closely the magenta layer fades at a much faster rate than the cyan or yellow layers, unlike the photo papers or inkjet where the colors seem fade more or less at the same rate. What does that mean? well, that was a problem for photo paper made back in the 60’s through the 80’s and it’s pretty common to find old color photographs from that era that are quite green.
Of course it depends on the purpose of the image, it’s not like the longevity is terrible, it’s actually quite good, and a lot of pieces aren’t intended as collectible art and getting a decade or two of use is great. Even up close the aluminum holds together enough for many.