have you downloaded the manuals & compared the features?
which way is "better" really depends on what you place the most value on & that's probably why it's hard for people to answer your question
for example, while the RF adapter for the remote might be interesting, the new receivers with their i-app control is important to many. if you want RF control, there are a number of options & perhaps better alternatives, like programmable remotes from URC. but if that feature is a must have, then a used SC-37 is a good choice.
what you will get with the newer models: DTS NeoX, more file media support, if you got an SC-79 or last year's SC-68 you'd get the async USB-DAC.
class D3 vs Ice amps...each one has its proponents. if you read past reviews in Home Theater Mag for example, for SC-37 vs SC-57 & SC-68, you'll see some minor test differences in power output, with the Ice amps being just a bit more powerful multiple channels driven at once, but the class D3 amps have lower distortion in lower to middle power range where most listening occurs. Neither one will probably be an audible difference but on paper, the class D3 amps have the advantage of simpler circuitry, not needing the feedback correction that Ice amps needed. you'll need to do some homework on this if you're a technophile type, much info has been posted over the yrs with links on the Ice amp models & the International Recfifier class D3 design, white papers, etc. I have the SC-09 and the SC-68 and the SC-68 has a very capable amps, holds up well.
speakers? I have Magnepans, 4 ohm, low efficiency. both the SC-09 & the SC-68 drive them OK. my gut is that the Ice amps in the SC-09 handle peak loads better, since the SC-68 has shut down on protect mode several time, with hi volume, very loud, high noise sound effect peaks, while the SC-09 has never shutdown. however, the SC-37 is NOT the equal of the SC-09 (7 X 200 wpc capable with its 1400 watt pwr supply) IMO, you would not notice any real world difference in power capability between a 37 and a 68/79.
in measurements I've seen online, the new amp design does seem to have bit less distortion at the extreme: high power - high frequencies. I suggest you read reviews of both on hometheaterhifi.com & hometheater.com
esthetics...some still prefer Pioneer's traditional gloss black over the brushed alum look. I'm one that does
but in the end, it's about sound quality & features, not the paint job
if you want to save money by getting a used 37 and seeking confirmation that it's the better choice, the only one who can answer that is you
I have no plans to sell my SC-09, valuing its robust build quality, unique construction & neat LCD display. but that doesn't mean I think it's automatically better than the SC-68, because in quite a few things, the 68 runs circles around the 09, like the new surround processing, PQLS over HDMI, having internet radio, the i-app control, and more.
it's up to you to decide how much, if any, differences between the 37 & 68 or 79 are important to you.
if I had hard to drive speakers, not sure if B&W 804 qualify, I think the 37 *might* be just a little bit more powerful, all channels driven, but at the expense of rising distortion. if you look at the Home Theater mag review of the SC-57, you'll see Ice amps have a gradual rising distortion level, starting at mid power levels, all the way up to clipping. IIRC, the distortion in class D3 amps is flatter in the mid power region, maintaining a much lower distortion level until it starts to turn upward, the curve being more similar to a class A/B amp. this is an advantage the new design has over the previous Ice amps.
you'll probably be happy with either choice, so do some homework, read the reviews, & decide if you aren't going to miss some features in a 68/67 or 79/77, like some of the file support, go from there