The biggest thing I have noted with using any and all software for radar is if you are in the field - range. Can you use the radar where you want to? What is your chase territory going to be. I don't want this to get into an XM vs. Grlevel vs. Swift debate - all companies make a very good product.
If you are going for "Eh, I'm only going to chase in and around major cities, or areas where I have data" then by all means, GRlevelX or Swift is the software for you.
If you are going to be out in the field, and you aren't bothered by sitting on the reservation waiting for initation, then look into XM, as you won't be bothered with "Am I within data range or not?"
When I'm in the field, I tend to use Swift, but when the going gets tough, or when data is spotty (Part of NE, SD, and KS) I'll be XMing it all the way.
Things I like about Swift: National Radar - don't have to worry about "Am I in range of this site or not?"
Position tracking - I personally think it's better in Swift, than on SpotterNetwork. JMHO.
Interface is also really slick. yeah, it doesn't have smoothing, but it also doesn't rely on the NWS servers as much. While GRLevelX relys on the NWS servers, or AllisonHouse's, Swift has their own set. So, when everyone is pounding the heck out of NWS's servers on a big day, Swift is on their own, and they have a dedicated set of feeds which, incase one is down, it will switch to the backup set without a hitch. You don't need to do anything - it just automatically does it.
Things I like about GRlevelX:
Very pretty. Quick interface. It integrates very well with AllisonHouses' feed and SpotterNetwork. You can write add on features for it with relative ease.
XM: Range baby, range. Doesn't have all the cool features of Swift Or GRlevelX, but it's got range. Middle of Candoo, ND? You got data. Sandhill Region, You got data. Sitting on the Res in SD, You got data. Also, when the Cell Phone towers get taken out by lightning, you still got data.
That kind of sums it up for me, but it's totally up to you - find something you are comfortable with, and something within your price range. All three companies make very good products, and each product has their die-hard fans. I just happen to prefer Swift and XM.
HTH,
-Eric