Tom, congrats on a new Helmsman 38E, that's a nice boat! If you're getting a new Garmin suite, it will be capable of SXM weather regardless of what you decide. The GXM 54 antenna can always be added at a later date if you push this off. I'm going through the same situation but I'm doing a Garmin helm update. the SXM antenna for this is almost exactly the same size as one of their GPS antennas, so it can be placed in the same kind of location on a radar mast or an arch; doesn't take much space.
We're going to go ahead and try it, we'll start with marine coastal, which is only $34.99/month and we can always turn it off later if we don't need it or are spending time off the boat.
By the way for those that also do SXM audio; you get a discount if you're doing both audio and weather.
From what i've seen I think we'll like having it, especially in places where cell service is spotty. Like I said, havent finished installing yet but we'll know in a few months....