Sobotka is more of a playmaker and can link plays up better than Berglund, and over the same amount of games last season as Berglund, he would've had 12 goals, not much less than what Berglund would get.
Adding a 20+ goal scorer would be great, but with our cap situation, it simply isn't realistic.
Stastny himself is a decent scorer that consistently gets around 25 over a full season, put him with guys that can play great on the cycle and win puck battles, and you have a successful line. Sobotka does that better than Berglund.
Sobotka-Stastny-Jaskin would be able to do very good things in tight spaces around the net because they'd finally have someone that can dominate the corners in Sobotka.
Sobotka has quality offensive skill, it just isn't his shooting ability. He's one of the best in the corners at winning pucks and keeping plays alive and is a quality passer and at times can make that great pass. He's a lesser version of Schwartz.
I've argued on this board that Berglund is still a quality player because of his all-round game, but Sobotka's all-round game is simply better. Our PK would be better with him, our center depth would be better with him, and Sobotka will get way more assists than Berglund, way more than the slight difference in goals.