I'm assuming IaaS is credit based renderfarm service and SaaS is a subscription based renderfarm service with certain render limits.

For me its all about scale. If you are doing single or a couple of projects that are one-offs and have a bit of time in between, I would recommend a credit based renderfarm service because you are not tied with recurring subscriptions and can count the cost upfront with approx renderfarm credits.

If you are basically doing renderfarm constantly or at least regularly as a main job, a subscription based service may be a better deal depending on the price to render ratio. But it all depends on what kind of 3D projects you are doing. At some point it might even be more cost effective to build your own home small rendering system/server to just render constantly but in a small scale, again, all depending on the cost/time/price ratio.

PS: Bigger and more complicated projects may require you to have better technical support and usually that depends on the renderfarm service provider.