n2ri wrote:also no price break for upgrading if you bought even a couple months b4 new brd came out
Because its not really an upgrade.... it was more a change that reduced the BoM cost for Solidoodle (and you could argue the swap to a 5.25mm barrel plug as the Sang couldn't support the depth of the socket... but this really was also to reduced the BoM Construction cost for SD (its quicker to plug in a jack at the QA stations than lug around a screwed in powerpack...). The MCU change is because that's what a printrboard ran and overall SMD is far cheaper to construct than through-hole as you can just pick-and-place then wave solder (what SD do) or Reflow 10's of boards at once vs hand soldering the through hole components (or DIP carrier...)
The SD card as shipped on the Printrboard is useless and probably will stay that way as they went and shared the hardware CS lines with the Y-End-Stops... meaning to even begin to use the SD you need to move the y-end stop to the e-end stop. Then theres the whole Marlin issue which remains largely unresolved due to coding snafu's... not to mention the 'dependency hell' that has started to creep into it...
Incidentally - you can add an SD card to the Sanguinololu using 4 measly pins that do actually work in firmware...
Or just plugin most LCD solutions, which all include an SD and solve the root reason most people want an SD card - unattended printing which is largely useless without an LCD which the printrboard wont supply you unless you use an i2c LCD which has its own issues again....
Which means the only substantive difference is the printrboard offers a PWM fan (that is both plugged up as a result of the wave soldering and also unpopulated but otherwise functional) header - via a pokey MOSFET anyway - which you need to do using an external MOSFET on the sang (but will ultimately use a superior MOSFET of a better rating anyway).
Anyway, There is no additional performance or otherwise benefit to the printrboard (not counting the "measly to fix for $5" issue of the PWM fan header that is present on a printrboard but not on the Sang) so wouldn't be worried they dont offer an 'upgrade' (since it isnt) - not even the SerialComms element as Marlin will choke on its 10KHz movement planner limits long before comms becomes an issue at 250,000bps (or even 115200 ....).. as thats a LOT of G-Code and Marlin already has a buffer....
There is the argument that 'as shipped', the SD Sang's were limited by their 644p processor vs the 1284p... but even thats a $5 DIY fix with no soldering and only an issue if you DO go ahead with any LCD mods and the like...
Functionally - both are ultimately identical in performance. The pros and cons of both boards from a hardware perspective pretty much also weigh up the same, with the caveat there is a lot more known's about sangs and their mods and capabilities than compared to a printrboard; which despite a long 'feature list' doesn't actually provide much more than a PWM fan header...
TL;DR : To tell which board you have... its pretty easy:
If your power connects with a Barrel Plug and the A4988 chips are mounted straight to the PCB - you have a printrboard. Rejoice or cry. You are in a minority when it comes to the modding world 
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If your power is screwed in to a screw terminal block, and the A4988 chips are on little 'carrier boards'... then you have a Sanguinololu. Rejoice. Mods and solutions abound, both here in Solidoodle land and just about every other printer forum.
the sanguinololu does have separate driver modules for the steppers which is a big plus if you happen to fry them or if you want to upgrade an axis to a finer microstepping