If I had to decide right now, who would I trade? I'd trade Greiss. I know Sharks fans won't like this, but right now this is probably the best solution. Sharks need a top six forward, and it's no secret that they do, but in order to do this, you're going to have to give up quite a bit. What player(s) are you willing to give up? Do you really want to trade away more draft picks because you're hardly in that position after the major trades with Minnesota? If you give up Greiss, you can dump in another player like Handzus and get a player like Ryan Malone with Tampa Bay or bring back Michalek with the Senators or even Ryan O'Reilly with the Avalanche. Actually, I like the O'Reilly fit because he's also a big body that can add some aggressive play that the Sharks lack from time to time. I think this also better serves Greiss who let's be honest got screwed over last year. By trading him to a team like Tampa Bay, Ottawa, or Colorado, he has a good chance at taking the number one spot or at least playing a lot more games. Handzus would also be a good fit for the Avalanche because he can add veteran presence to a young team.
If you trade Niittymaki, you might be giving up Jamie McGinn. I think he could potentially have a great career with the Sharks, and I think his potential is greater than Greiss who probably would want to be leaving the team in a couple years anyway since he probably would want a starting job which wouldn't happen under Niemi. I think the Sharks don't have enough young talent quite yet to be trading away a player like McGinn. Even if you don't want to keep Niittymaki, there are plenty of other viable second string goalies. Niemi takes the bulk of the workload and needs it, so it would be more beneficial to get a Giguere or Garon since they wouldn't have to play as often. In fact, Giguere might even prefer it, but you might have to pay him more than you'd want. Garon you might not have to pay too much for which is beneficial since this isn't a position you'll want to focus too much of the salary cap.