There's been a lot of generalized arguments for and against Steam in this thread. Let's instead look at the specific case of you migrating your copy of CK2 from GamersGate to Steam.
1. With GG, you needed to download and run the GG installer of CK2. It asked for your GG login information and required internet access.
- With Steam, you need to install the Steam client. It asks for your Steam login information and requires internet access. It is intended to be a multifunctional piece of software and accordingly is larger in size than the minimalistic GG installer (a full install of Steam is ~400MB), which can be seen as bloat if you don't plan to use the additional functions. However, this shouldn't be a major concern unless you have very limited disk space.
2. With GG, after installation you can launch the game without having internet access or running external third party software.
- With Steam, after installation you can launch the game without having internet access or running external third party software. No difference here.
3. With GG, the process of applying a patch would go like this: you note there's a new patch in the these forums -> you investigate and decide if you want to install the patch at this time -> you download the patch installer and click through the installation wizard -> you close the installer -> you launch the game as usual.
- With Steam, the process of applying a patch would go like this: you note there's a new patch in these forums -> you investigate and decide if you want to install the patch at this time -> you start Steam client and let it download the update -> you close Steam -> you launch the game as usual.
4. With GG, when installing a new DLC the game would require internet connection to contact GG servers and authenticate your DLC.
- With Steam, when installing a new DLC you would need to launch the game once through Steam so that the game could contact Steam servers and authenticate the DLC.
5. By default, GG sends out newsletters containing targeted advertisements based on your past purchases. This behavior is disabled by unticking a single checkbox.
- By default, Steam client opens popup windows with ads for new games on Steam store. This behavior is disabled by unticking a single checkbox.
6. To use GG, you need to provide your email address, name and personally identifiable payment and billing information. GG knows which games and DLC you have purchased and when.
- To use Steam, you need to provide your email address. Your name and personally identifiable payment and billing information are only required if you intend to make purchases directly through Steam, as opposed to other retailers that sell Steam keys, e.g. Paradox Web Shop. Steam knows which games and DLC you have purchased and when. It does not know how long you play them, or what you do while playing, or information about your computer's hardware unless you choose to always have the Steam client running and opt in to the hardware survey.
It is entirely possible that Steam won't offer any improvements for your personal usage habits and preferences. (That is to say, you will not see benefits in the additional features that other users may find useful.) However, I don't see how it would worsen your experience either - certainly not to the point where you are perfectly fine with GG but categorically against using Steam.
