The question is .... what is the best way to get a plan and phone? Online? The Store? Costco?
If you are looking for the best price on the device, then online is the way to go. You can certainly buy from the Verizon site, but also have a look at Wirefly, as these places offer about the lowest equipment prices I've seen. Most of the time the phone and even some accessories are free.
One word of caution, though. When you shop from an agent (any company besides the carrier), read the terms carefully. There are sometimes restrictions on lowering your plan within the first six months, with significant penalties if you do. However, the easy way around this is, when you initially sign up, get the cheapest plan you can, within reason. Then, if that doesn't meet your needs within the first few weeks to a month, move up to the next one. You'll incur no penalty this way. Also, read the terms related to canceling your service before your contract ends. You will owe the carrier an Early Termination Fee (ETF), but you may also owe one to the dealer you purchased the phone from. This isn't always the case, but just be aware of it. The items I've mentioned in this paragraph can apply anywhere other than the carrier's Web site, stores, or telesales. You can encounter this at any third-party agent, so if you have any doubts, ask questions and read everything you're given.
And I have to mention rebates. You're going to encounter them all over the place. If you plan on making that a big part of your buying decision, you need to understand how tricky they can be. I've seen terms that state that you have to wait six months before you can file the paperwork, and you only have a one-month window to do it, or the rebate is void. These things sometimes appear designed to cause you to make a mistake and not be able to redeem them, so be warned.
You may consider looking at other carriers too....other than Verizon. So if you look at other options, and still like Verizon, nothing wrong with that. However, if you haven't, then you should consider all your options. For example, AT&T offers about the same coverage as Verizon in many areas, but their 3G footprint is much smaller right now. However, they use GSM. So their devices work much better overseas, since GSM is the dominant wireless standard around the world. Verizon, Sprint, Alltel, and some local and regional carriers use CDMA, which works well in the U.S. and Canada but not so well (or not at all Cellulari TRIAL 3 SIM) in other countries. There are some CDMA devices that also have GSM for international roaming, but they are limited. If international usage is an issue I'd suggest looking into OneSim.
As for Sprint and T-Mobile, my friend had Sprint back in 2001 and 2002 and found their network to be quite weak. Especially inside buildings, with enough dropped calls to almost drive him insane. That may have changed .... but they aren't hitting any high marks in customer service right now, and my understanding is that they're bleeding customers. There's virtually no chance they'll cease operations, but they seem to be in a state of disarray. So, unless you need a really cheap data plan, then I see no reason to choose them. T-Mobile seems to be a good carrier overall, but they don't have much in the way of 3G service. And because of the frequency that their 3G network uses, only a few devices can access it. However, like AT&T, they use GSM, so your device will work well overseas.
One last thing. I've mentioned 3G a lot. Having it is very useful if you are going to be using lots of data, such as getting maps and transferring lots of files, but it really isn't as critical if you only plan to check the occasional e-mail.