I went through the IS at the start of the year and remember it being a fun, mobile tank. It was pretty easy to pull out some big damage especially when top tier. Now that I'm working through the IS-4 line, I'm just about ready to let go of my KV-3 so I've got pretty recent experience with both. I'd say you have to base it on desired play style. IS is far more mobile so would probably suit your medium style tastes better but once fully upgraded and you've got side scrapping down the KV-3 is a game carrying beast. I also find I can hold my own far better in the KV when I'm low tier in a match and can hold the line against most higher tiers. Also have to say I'm much happier to see an IS on the enemy team rather than another KV-3 as I don't find the IS intimidating anymore.
I thought I remembered hearing that the IS had been nerfed in a recent update. I only noticed as I swear my stats on that tank have been changing from dark green to blue recently. Would this be based on tank averages dropping? If so, it's either due to a recent nerfing or the lack of depth in the player pool lately as I haven't owned the tank in months.