Kinda depends. Sirius works with Kenwood, while XM is with Alpine and Pioneer. Look at what available. Technically, I believe Sirius gives you more channels and more commercial free channels, but I'm not positive. Its also still being built, so it might not yet be available in your area. Prices should be similar.
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My Xterra Pics
I wish I would have a real tragic love affair and get so bummed out that I'd just quit my job and become a bum for a few years, because I was thinking about doing that anyway.