If it is important to you, your magna probably is faster than the VTX. It won't have the power, but it will probably win stop light to stop light.
I recently bought my VTX, so here are some of the reasons that went into it for me:
First, I sold a Valkyrie Tourer last year. I regret it. I looked pretty hard at getting a used Valkyrie, but I decided on the VTX because of the riding position and the feel of the motor. They have similar power, but you get more of a sensation of power with the VTX.
I also looked at the Kawasaki Mean Streak and VN2000. Kawasaki definately makes a quality product and I think their attention to aesthetic details is superior to Honda's. The mean streak looked and felt too small to me, though it gets very good reviews. The VN2000 is just huge, and not in a good way. It is heavy to get off the side stand, heavy to try and push around with your feet, feels awkward. It also has a really low red line, so you have to shift a lot in low gears. I think a mean streak version of the vn2000 will be a winner though. Its also a first year model and that makes a bit nervous.
Yamaha's bikes have a lot less power. They are really sharp looking in a lot of ways and the aftermarket is developing impressive things for them. But...power.
I took a look at the Victory bikes too; they have some cool stuff going on at Victory and their bikes are really sharp looking. They have also been very cleanly designed with a lot of attention to details...the problem was that the cheapest bike they sell (at least that I was looking at), was the 8-Ball and it was still almost 2x the price I paid for my bike.
Ultimately, the VTX1800 has:
A big motor with lots of power. It also reves high enough that you aren't always on the shifter.
Cool feel; at idle, you can feel those big honking pistons thumping beneath you...this sold me on the VTX over the Valk. The Valk was a sweet, sweet bike, but after several years...I really hate to say it, but it was almost boring to ride.
Lots of aftermarket developing. That was another thing about the Valkyrie; there wasn't a huge amount of aftermarket. The most exciting things that came out for it were developed by Lamont and other talented guys on the Valkyrie owner's board.
Comfortable riding position. (although the seat makes my back hurt after not so many miles...got a new one on the way)
Lots of nods from people that see it; even when trailoring it home after buying it, I had people stopping me at the gas station wanting to check it out. Almost nobody cared that it wasn't a harley, which is refreshing.
Fuel injection is cool. Its nice not having a choke, not having to mess with it in cold weather. No altitude changes...just hit the starter and off you go.
Finally, if you can get the new old stock bikes (i.e. 2002, 2003, 2004) models that are still around, you can get an amazing price. I paid about $8600 for mine out the door, thats about $3k less than dealer invoice when it came out in 2003. For that kind of savings, I've been able to add custom stuff to make the bike fit me better and still come out ahead...can't be that with a stick.
Things against the VTX
Its a big, heavy bike. Much heavier than your Magna. That takes getting used to.
It scrapes a lot going around corners.
Not as nice details in how the wiring etc... looks. You can fix this yourself, but other makers are much better at it than Honda.
The seat is a bit annoying to get off...I'm used to removing it with a key...now I need a wrench.
Stock, I was not happy with the exhaust. Very quiet and very mechanical sounding...not much soul to it.
On the older bikes, you still have the tank seam (I think every 2005 bike now has a seamless tank)...I hate the tank seam. 
I hope this helps.
There are a lot of cool bikes out there, but I think Honda really put together a sweet package. Its a lot of bike and for the prices some of the older machines are going for, you can get a heck of a deal on it. Compared to the competition, the prices some of these bikes are going for make this about the best deal out there.
Good luck.
