Now that I have handled both as a daily driver for about the same amount of time, I can share some of my opinion on comparing both experiences. (Long post ahead, but just trying to be helpful. For reference, I'm a moderate user. I rarely play games and if I do, it's word search games. I browse sites, use social media, read ebooks, and occasionally take pictures.)
Anyway, lets go into my personal experience with each by category, starting with the short and sweet version.
Short and sweet version of the detailed points below and who wins for each one:
Camera: 550 wins by a slight edge because it works in all environments generally (daylight and low light)
Connectivity: 550 wins because of both LTE and WiFi connectivity
Sound: 550 again, because it's louder and ringer isn't as silent on top of paper and cloth
Screen: BLU wins because it's sharper, prettier
Battery life: Tie
Feels in the hand: Subjective really so N/A
Speed: BLU because better processor
Buttons: Subjective really so I can't give a score for that
Now for the detailed version:
Cameras:
-As amitnahar said above, the BLU's biggest downfall is this alone. I have found that OneShot can help this situation with both to an extent. Low light pictures (like ones at nighttime and also indoors) aren't good. In my experience, those shots are just very dark. I have tried being near the light and there's little difference, especially for the front shooter. The flash gives off a very blue cast that almost makes the scene look bluish. Video is also dark by the way too in low light.
-The 550 on the other hand, has decent shooters, both front and back. For low light pictures, there is something that makes the pictures look a bit yellow at the start, but if you give it a little time to adjust, the color will stabilize to a normal one. As for the front, the yellow is very noticeable instantly. It does a weird thing where after taking a picture, you see the color appear to be more yellow. Just like for the front camera, occasionally if given time to adjust, the color can improve somewhat. However, this improves a little bit more instantly if you set the ISO to 100, or the shutter to 1/8. Oh, before I forget, video quality for the front camera in low light, is very noisy/grainy looking. You can at least see yourself, but it just wouldn't be a good experience.
Connectivity:
-The BLU has great LTE connectivity. The speeds appear to be slightly faster than the 550 in all three of my speed tests while connected to WiFi. Speaking of WiFi, the connectivity on that front personally annoyed me, since it was always dropping. I have great WiFi at home, but I never saw that reflected on the BLU. In short, data connectivity = great!, WiFi connectivity = meh.
-The 550 has all around great connectivity on both WiFi and LTE. I have also tested this outside of my home (like in an another area altogether and at home, I did the same with the BLU, by the way). There's really nothing else to say here.
Sound quality:
-BLU: Just okay, really. Watching a video on my phone with the volume was okay if I was holding my phone and watching, that is. Placing it on top of paper or cloth and having the phone ring is a disappointment. Someone called me and I didn't even know the phone was ringing at all (in a quiet room for that matter). The sound of calls is decent, it does the job. Speaker phone on the other hand, is alright.
-550: The sound quality is good enough. It's loud enough to be heard but it's not amazing. I haven't tried speaker phone though, so I can't speak on that. If you have a 640, you would know about the weirdness of having something like a video, music or a phone call seem more quiet on top of paper or cloth. The 550 does the same thing, but I'm not sure of what extent right now (as in if it is the same, better or worse), I'll have to test that sometime in the future.
Screen:
-BLU: Wins hands down. It's beautiful, big sharp screen is great to look at and read on. Whoa. (Even better than the 640 honestly)
-550: It's comparable to the 640's screen. That basically means that it's pretty good, just not as good as the BLU.
Battery life:
-BLU: Good on battery life and normally I can get a day and some on a charge.
-550: It's also good on battery life. It seems about the same to me, I guess (Win 10 Mobile is a work in progress, so I have been messing around with background apps, brightness, etc). In short, not the phone's fault but the OS really.
How it feels in the hand:
-BLU: Feels great in the hand. Even though it is a 5" device, it doesn't feel like it. It's lighter than the 640 is.
-550: The size of it makes it feel really good in the hand, even though it's thicker than the BLU is. Can be used with one hand easily.
Speed/how it runs:
-BLU: It runs much snappier, and a little faster.
-550: It doesn't run slow, but it isn't as snappy as the BLU. I don't feel like I'm compromising in speed.
Navigation bar/buttons:
-BLU: Physical buttons that light up without feedback
-550: Software buttons that can be hidden but have feedback
Side notes:
The BLU has more color choices (yellow, white, grey), which is always nice. The 550 only comes in black and white, but Mozo has made back covers in green, orange, and blue. You have to do a workaround (check out General Windows Phone Discussion Lounge to find the thread) to install HERE maps on the BLU, and no MixRadio is available regardless. MixRadio is available on the 550 but HERE maps isn't. Microsoft maps is pretty decent though and uses HERE maps data.
I have no picture samples. Between the two, I would say that it depends which one will better work for you day to day, and just in general. If you like to video chat on Skype with either device using the front camera, doing so with daylight will be your best bet. However, if you must do so in low light, the 550 will provide that experience for you, even though the image quality wouldn't be great. If you are not a big selfie picture taker (as in, you rarely/never do it), I would say the 550 would be the better bet.
The BLU is great for watching videos, and playing games with that processor and big, sharp screen and generally whatever else you wish to do.
You can't go wrong with either one really, but in case you are a person who does the low light experience (I am kind of a concert head, so the low light experience is a consideration for me), the 550 would do it at the lowest price point.
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