1) Blu-Rays will usually have better quality picture than an HDTV broadcast. However, comparing to BD rips, there's usually not much difference. Groups that are doing those tend to be doing it specifically for the picture quality, so they both tend to know what they're doing and tend to avoid shortcuts and skimping on things.
2) Translation quality is all over the place, and is heavily depending on the series itself: specifically, the things professional translators tend to fail with and the groups that are subbing.
Professional translators are often either overworked (yes, I know some) or simply unmotivated to look heavily into things, and have four faults they typically exhibit:
-The first is words and phrases in a third language. I've seen this screwed up or even just not bothered with time and again, often relying on my own knowledge or research skills to figure things out - anyone who thinks this is never a real issue is invited to look at the booklet for the US release of the GitS SAC OST vol 2. Be prepared for massive facepalm. Granted, not quite a series itself, but still an excellent related case-in-point.
-The second is related to the first. If there's something technical in there or references to other works of fiction, it's not unfathomable for a translator not in the know to get lost and screw up. Granted, some weaker fansub groups do the same (I recall one of the Gundam Seed groups using "AVAX" instead of "AWACS"), but you'll rarely see that in a competent fansub group.
-The third is the inability to provide explanations. In my vast list of DVDs and Blu-Rays, I have less than 5 series out of 120+ with decent liner notes, and there are a number of shows that could really,
really use them. Thankfully, for those, there's often been Shinsen-subs. Seriously, anyone who thinks fansubbers can't eat the pros for breakfast here should see their post-episode liner notes for shows like Rental Magica or Demonbane.
-The fourth is that they often have a desire to keep things somewhat different, and I've seen a number of titles where an almost universally accepted translation of a popular phrase was changed to something with a similar meaning, but drastically inferior and
clearly a worse translation, with no other reason I can think of than, well, not wanting to use the fansubber version.
-A fifth one that may or may not be a plus, depending on one's views, would be the greater leeway fansubbers have in what they can put on the screen. Sometimes helpful TL notes, sometimes just hilarious references, or even goofy translations (as Commie's been doing with Girls und Panzer) that you'll never see in official releases. These can significantly increase the enjoyment factor in a series - if you like them.
As far as fansub groups, it's important to differentiate between CR rips, traditional speedsubbers, and quality subbers. CR rips just take the subtitle scripts from Crunchyroll and mate them with higher quality video files. Note that CR's subs are actually official subs from professional translators not unlike those doing your domestic DVD/BR, but are known for often having relatively poor translations, and thus are often shunned by "purists." Traditional speedsubbers, who've been pushed away to a large degree by CR rips, are out to get you a translated product as quickly as possible - they tend to skimp on making sure the translation is just right and other quality checks, but they come out quickly. On the other hand, quality subbers often take awhile to release their product, going through several layers of translation and quality checking before being satisfied that it is a good project, and even researching things they're not sure on (like those phrases in other languages and technical terms). These can easily end up being the best translations you'll get, professional or otherwise.
Note that a number of DVD/BD rip groups actually rip the official translations as well.
Note that little things like spelling and grammatical errors can be found anywhere, including high-end translations. As a writer myself, I catch them regularly.
All that being said: buying the Blu-Ray marginally helps the Japanese industry and significantly helps the industry in any other nation. This is a good thing. Buying lots of official stuff is good for everyone, so I encourage it. Also, you could always end up owning something like
this or
this, or maybe
this, and get to tell people how you only paid a fraction of that when you got it (actually, I had 2 copies of City Hunter 3, and ebayed one for over $150 when I really needed some cash 4 years ago)!
