I'm late to the discussion, but reading Umbran & Jester discuss plot, I think has helped me understand my displeasure with the movie. Thanks to both of you for stimulating my ideas.
SHORT VERSION:
The First Order SHOULD HAVE ATTACKED the Resistance Base.
That's using hindsight and viewer-knowledge.
You attack the Resistance base first only if you're more worried about short-term tactical threats. And they weren't, because as far as they could tell, there was no threat! Starkiller Base was perfectly safe from Resistance assault. The Resistance could be safely ignored for the larger strategic goal. Except that Han Solo conceived and executed a piloting feat nobody does - coming out of hyperspace *that close* to a planetary body. It just isn't done. It is as if Han pulled a secret weapon out of his butt, and you can't expect the First Order to have considered that possibility.
Now, why is the First Order jumping back and forth like that? That's simple, when we think about it....
It seems like the First Order (and by extension, Snoke) wants to destroy the Republic and take over, right? Do you want to do that with a protracted war? No! Starkiller Base is a way to do that quickly and cleanly - destroy the capitol and then while the Republic thrashes about with it's head cut off, mop up and move in. Simple, clean, effective. To be honest, maybe Starkiller isn't mobile - it may only really be intended for one shot.
Except for one thing: Starkiller or not, a Jedi probably poses the only credible threat to the plan, and Snoke's life. Snoke has learned from Palpatine - being in power only makes you a target of Jedi. Snoke's best bet is to not be too much of a threat until *after* he's killed Luke. He can't even use Starkiller without alerting Luke - Jedi can sense the death of a planet over interstellar distances.
But, the Resistance getting a copy of the map throws a zero-grav spanner into the works. Major attacks on bases will not assure that the Resistance does not alert Luke to the situation - one guy in an X-Wing slipping by, and the jig is up. The plan only survives by retrieving the map before the Resistance has seen it. There's some thrashing around trying to do that, but ultimately Kylo Ren fails to accomplish it. Then, there's no point in waiting. Snoke must assume Luke is going to find out one way or another. He sets off Starkiller, taking out the highest priority target first, and pulls Kylo Ren back for training, because that Jedi may take a while, but is on his way....