GSCE and rFactor 2 are great sims, and as Ole points out there are still quite a few leagues that are running rFactor 1, so from a community point of view it largely depends what kind of mods and/or leagues you are involved with.
If you already own a copy of rFactor 1 I would definitely consider getting a Steam key for it. If you don't, there are a couple of new features that might make the Steam version worthwhile. Obviously the integration with the Steam platform is worth mentioning. If you host a game through Steam, it shows up in their "matchmaker" which can be seen if you open the "Servers" list from the menu in your Steam client. That also means you can easily jump into the same on-line session as your Steam friends (right click and join). Stats and achievements are a nice touch (sure, you can debate if that really adds something for the serious simracer, but it's always nice to compare achievements with your friends). Things like voice chat, and the ability to broadcast video while you're playing are nice, and we added support for the Steam Music Player, which can be mapped to specific controls on your wheel or button box to simulate a "car stereo".
We also took the opportunity to add some features that are not specific to Steam. One is support for 4GB. rFactor is a 32 bit application and out of the box it supports up to 2GB of memory. More recently, big mods and big on-line fields were reaching that limit. Out of the box, 4GB are now supported, which means that you can use large fields of cars and mods with many different models. Also, rFactor had a controller limit of 3. That has been increased to 6 to support people with extensive cockpit setups. At the same time we increased the number of buttons per controller from 32 to 64 (to support large button boxes). Also, for people with triple screens that want to run "realistic" vertical FOV settings, the minimum FOV value now goes down to 10 degrees (from 34) and you can assign buttons for changing the FOV in your cockpit, which makes setting it up a lot easier.
In the end it's up to the individual simracer to decide if this is worth it or not. ISI has made it very clear from the start that they chose rFactor 1 to "test the waters" so they could make an informed decision, from a technical point of view, about also bringing rFactor 2 to Steam. Some people like the convenience of having all their titles in the Steam library, others simply don't like Steam. You have a choice now and I guess that is always good.