I asked this same question a few months ago...If different HUs with the same features sounded different...
I found that one big difference in HUs is if they are 2way, 3way, or 4way. Which one you choose depends on your setup. Think ahead of what your completed system will be and choose wisely, you dont want to replace your HU later on when upgrading bc you need a 3way or 4way xover that you didnt buy.. Tweets, Comps, and sub(s) would mean you want a 3 way HU. front/rear coaxials and you want 2 way. Tweets/Front/Rear/Sub and you want 4 way.
The next function difference is the EQ. You should really look into this before you buy. Some have 3 bands parametric eq, some have 5. Some just have bass/treble controls.
Look at the DAC of the HUs youre looking at. you want 24bit burr brown, but 24 bit would be ok as well.
Another thing to consider is if you are fine with playing only store bought cds, want mp3 cds, a 3.5mm jack, a usb port, or all of the above. For example, nakamichis are great sq that you can find for under 200, but wont play anything but original cds/3.5 aux. Maybe you want to have a series of 16gb thumb drives in your glove box? For some reason, many of the higher end (3way) HUs (alpine/pioneer) require you to buy an external cable for that, or just dont support it.
I wanted great SQ, 3way xover, 24bitDAC, >4v preout, and usb for under 200 and i ended up with a clarion dxz775usb (nib
ebay-120shipped)...and am extremely happy with it. Out of the box, it did require some tuning of xover, but tuning is half the fun. If you want bluetooth the dxz875usb has that. Some people dont like the simple colors of it, or they want knobs instead of "track buttons" or whatever theyre called. The next step up I saw was a Pioneer P-860 for around 220. You get the knobs and OEL color display, with all the features I listed, but have to buy a usb dongle (that has good reviews and works as opposed to alpines). The issue here is that these are all pretty obscure HUs, that youd have to wait on
ebay to find.
With the new Pioneer P80 out, I think you might see some used eclipse 7200mkii's around, which have all the features, but youre looking at 320+ for a used HU there.
If you dont care about usb...that clarion drz9255 is pretty sweet.