The Ark (whatever its final name will be) is not the Galactica, and we will not be fleeing from Cylons who will chase us. I further posit that there will be no Cylon civil war resulting in the some Cylons siding with humans while other Cylons remain intent on destroying humanity while led by Brother Cavil. And I'm equally sure that we won't be going on a search for Earth in ME:A (one that develops increasingly mystical overtones) while led by Angel Starbuck who ultimately goes poof in the middle of a field after the crew of Galactica lands on primitive Earth. We also probably won't be making any boneheaded decisions to throw away all our technology to live low-tech agrarian lives in the hopes of avoiding repeating past mistakes.
So no, I don't think ME:A is a spinoff of BSG at all. Short of Tricia Helfer voicing EDI in ME3, I don't see any direct connections between BSG and the ME-verse at all. The idea of fleeing some horrible disaster with only a few surviving humans on an ark dates back to the whole story of Noah's Ark, which in turn takes its ark idea from Gilgamesh, an epic that also happened to involve a great deluge. The whole catastrophic flood thing is a motif that you see repeated in myths from multiple cultures, so it's not unique to BSG.
As for the geth/quarian thing in the ME trilogy, I'll just point out that the idea of robots/synthetic life as slaves dates all the way back to R.U.R. by Karel Capek, who first used the term "robot." A lot of works of fiction can be said to be derivative of past works in their respective genres, and in BSG's case, the original series was pretty clearly meant to capitalize on the Star Wars craze at the time (and there was actual legal wrangling over that in the past).
However, I don't see that much in the way of direct 1:1 similarities between BSG and ME:A to the point that I would call it a clone of BSG. What I will say is that most ark-type stories probably owe a lot to Gilgamesh and other epics/myths of their ilk.
Also, "spinoff" was the wrong term to use as far as I'm concerned. I would probably have been slightly less annoyed by the thread title if you had said ME:A riffed off of BSG instead incorrectly using the word "spinoff," OP. See, a spinoff is essentially a child series based directly on (and with ties to) its existing parent series. In other words, shows like NCIS: New Orleans and NCIS: Los Angeles are actual spinoffs of the original NCIS series and share the same universe and characters who occasionally do crossovers.
I hate seeing terms used that I consider incorrect and/or imprecise - hmph.
EDIT: Just to elaborate a little more on this, Agent Pride (played by Scott Bakula) who leads the team on NCIS: NO first popped up in an episode of NCIS. His character was later spun off into the NCIS: NO series, which I watch partially because I like the main series but mostly because I loved Quantum Leap (which also starred Scott Bakula). Since a Quantum Leap reboot seems to have zero chance of happening at this point, NCIS: NO is as close as I'm getting to seeing Sam Beckett anytime soon. I was definitely amused by that one episode with Dean Stockwell as a guest star, since that was as close to a Sam/Al Quantum Leap reunion as I'm going to get.