Glocks are the Toyota Corolla of the handgun world -- inexpensive, simple, ugly, utilitarian, and reliable tools that run forever with only a minimal amount of maintenance. I would consider going from a Sigma to a Glock to be maybe 25% upgrade and 75% lateral move. If that's what you're looking for, go for it.
But if you really want to upgrade you should look at Sig's, HK's, Browning's, Beretta's, CZ's, 1911's. Even the new SW M&P has gotten good comments from a lot of people. If I were looking to upgrade from a Sigma, there are lots of other guns that I would put ahead of Glock as my choice.