If you plan to just play games, get the premium. If you plan on renting movies or downloading TV episodes then the extra HDD space on the elite may fit you. I wouldn't recommend an arcade to anyone unless you really really like the XBL arcade titles.
I want to get the Kids an Eilte but they are impossible to find here, then again only the Arcade is in abundance here sadly. I refuse to get that so if MS does not get it's act together and ship more Elites and Pros I'll just pass on that idea all together. If I had a choice though it would be an Elite
Dude even here in the middle of nowhere our Best Buy has a crapload of elites. Well they did about 2 weeks ago. edit - Even online places are sold out? I don't know if Newegg carries consoles but you could try there. Walmart has site to store which is nice also.
It all depends on how much you want to save in the long run. The Pro is the best way to go. It comes with the 20GB HDD which is $99.99 by itself, HD AV cables (which also includes regular composite cables), ethernet cable and wired headset. The Elite is exactly the same except for the obvious black color scheme, 120GB HDD, and included HDMI cable that is areound $50 by itself. And both the Elite and Pro come with Forza 2 and Marvel Ultimate Alliance. The only thing I see justifying the purchase of the elite is the color and/or the 120GB HDD which is great if you're downloading a lot of games, movies, shows, ect. So if you're not planning on downloading a bunch of stuff don't bother with the Elite. The Arcade bundle is not worth it at all, yes it does come with a wireless controller and 5 arcade games now but there is no HDD, no headset, no ethernet cable, and it only comes with composite cables for regular definition tv's. Spend the extra little bit for the Pro and save a buttload in the long run.
Also the premium systems now have HDMI built in like the Elites. They snuck this feature in without saying much. If you have a TV that supports HDMI you don't have to buy the elite for that now either.
If going for the "Go Pro" premium or "Go Large elite bundle, look for the following on the carton. 1. make sure you have a console with the 2 free games packed inside. 2. look for a lot number (above the SKU number) of greater than 739 3. if the system is of lot number 739, look for the wattage rating on the box. Make sure it's at 175w. If you find something like this, your console will have the newest chipset. A cooler running 65NM core will be powering the system. This will effectively end the "red ring of death" which plagues all existing systems to date.