Well, the main downside to the X51v right now is the fact that Dell no longer sells them and I'm not sure about the support. You can likely pick one up on Ebay or on one of the Axim fan sites, but they will all be a minimum of a year old. I've had mine for a few years now and love it. 640x480 isn't as nice as the Nokia's screen, but the smoking 624 mzh processor is sweet. The addition of a dedicated graphics processor (Intel 2700G) makes gaming and other graphic-intensive tasks manageable (as long as the code leverages that power).
Another downside is no support for WM6. Windows Mobile/CE/whatever justifiably had a bad rep. WM5 was a good step forward. I've been unable to use WM6 since Dell never supported it.
For those who mourn the use of IE, I know of no one who uses it. I use Opera and there is also Netfront.
Add to the "pros" column the fact that there is more software available for the Dell than you could ever want. Nokia is helped by the Palm OS emulator, but I have one of those on my Axim that runs very, very well.
Each are good devices and fill a niche. I think you'll be happy regardless of your choice. Since I own one, you'd think I'd stop Jonesing for an iPhone and an 810. Yet, here we are.