Yoda had been learning force techniques in exile, but, he hasn't been practicing the combat aspects, where as the darkside is all about power and Sidious has been studying for 20 years.
That's the way I had always imagined it, so take it for what you will, seeing as how there is no proof and it is just my personal opinion.
I think Sidious could take him. He simply outclasses him.
Wait, this is ROTS Yoda, who is in his prime, against OT Sidious who is way past his prime. I misread that thinking this is Yoda right before he dies. In that case, I think Yoda would take him, since he's a lost quicker, and Sidious although he may have studied the dark side(mainly the cheating death technique we see from Exar Kun), I don't think he's studied anything dealing with combat He's old wrinkly and slow.. Yoda owns him.
And you're getting this where? Yea I've heard of his so called Dark Side Compendium, but I'm in no mood to argue whether the sources are canon or not, which most likely they aren't. Everything else you just said, you might as well back it up. The only thing he learned that we are sure of is how to cheat death, Exar Kun style.
Usually when you study such knowledge, you get more poweful. Look at... Exar Kun with Sadow's stuff. Revan and Kreia with Malachor's stuff.
You learn more techniques, and practice makes you more powerful. Palpatine was so obsessed with the Dark Side he left most of the "Empering" to Vader, the Moffs and his advisors. He did practice the Dark Side alot. It's more than safe to assume he's more powerful.
Believing the Sids didn't get stronger by RotJ is retarded. No Sith Lord SANELY stays out of practice, especially with another powerful Sith Lord constantly conspiring to kill him. Between training Sith Acolytes, and manipulating and corrupting worlds and their leaders, Sids was constantly relying on the Dark Side, and he grew more powerful to the extent that in DE he stated that his body began to degenerate and rot away. That is Power(though has it's side-effects).