Not rental car specific, but my wife and I have two vehicles that both have SiriusXM. One of them - a 2011 Kia Soul - gets amazing reception and hardly ever cuts out. The other - a 2015 Nissan Frontier pickup - is not nearly as good on the reception and seems to cut out nearly every time I go under a bridge or even if there are trees along the side of the road.

The way I've heard/seen it explained is that part of the difference in signal is whether the radio is a Sirius system or an XM one. Radios that are Sirius branded have better reception because apparently there are more towers/satellites/etc on the Sirius side. XM-branded ones don't work as well because there aren't as many towers/satellites/etc. My Kia has a Sirius radio, the Nissan has an XM one, so this theory checks out in my experience, but YMMV.

I don't have any info on the free trials except I think both of our vehicles came with a free trial when we first bought them, but things may have changed since then.