Reader mail: Should I get the 16 GB or 32 GB iPhone?Margaret writes: I’m finally ready to break down and get the iPhone 4, but here’s my question: which one should I get, the 16 GB version or the 32 GB one? I don’t want to spend an extra $100 on the 32 GB iPhone if I don’t really need all that space—but I’m also afraid of running out of memory. Thanks!

Greetings Margaret—and good question. But first, I have a question for you: What are you planning to store in your iPhone’s memory? A few apps, albums, and a music video or two?

Or are you hoping to watch entire seasons of “Glee” from the iTunes store, or download the latest cutting-edge games?

If you’re not all that keen on watching TV shows or entire HD movies on your iPhone, you’re probably safe getting the $199 16 GB version rather than the larger, 32 GB model for $299.

An individual MP3 song file averages about five megabytes in size, so given that a gigabyte is roughly 1,000 megabytes (well, there are actually 1,024 MB in a GB, but we’ll call it a thousand for simplicity’s sake), you could probably cram about 3,000-ish songs—or more than 300 full CDs—into the flash memory chips of a 16 GB iPhone 4.

E-books, applications, and snapshots tend to be pretty lightweight, too. Your average novel on the iBookstore weighs in at a mere 1 MB—less than a fifth the size of your typical MP3 track—while most iPhone apps are less than 50 MB each. (The wildly popular Angry Birds, for example, is just 18.8 MB). Photos snapped by the iPhone 4’s five-megapixel camera, meanwhile, are only 1.5 MB or so, meaning you’d have to be quite the shutterbug to put a dent in 16 GB of total storage.

So in that case, you might be asking, why would anyone ever want a 32 GB iPhone for $100 extra? One word: video.

A 45-minute, standard-definition TV show from iTunes gobbles up a whopping 500MB of storage, while a two-hour SD movie typically takes up more than a gigabyte. Like to watch your movies in HD? Get ready to reserve about four or five gigabytes for a full-on, high-definition feature film. And if you’re a budding Scorsese who wants to shoot oodles of mobile HD video, watch out: just 10 minutes of footage from the iPhone 4’s 720p-quality video recorder takes up more than 800 MB of memory.

One more thing: While casual mobile games like Angry Birds won’t take up a lot of storage space, some of the more popular, graphically intensive games in the App Store—like, say the cutting-edge Infinity Blade—can take up more than 500 MB of space each.

Of course, you could always conserve space on your iPhone by only syncing the apps, music, and video that you absolutely need and keeping the rest on your PC or Mac; then again, all that syncing can get to be a hassle, particularly when it comes to gigantic video files.

So, here’s the thing: If you only see yourself syncing a few hours of music, a dozen or so apps and games, the odd snapshot, and an e-book or two, then a 16 GB iPhone should suit you perfectly fine.

But if you’re a video fiend or you’ve got an itch for the latest processor-busting games, you might want to go ahead and spend the extra $100 on a 32 GB iPhone.

Hope this helps—and if you have more questions, let me know.

P.S. If you’re in the market for an Android phone rather than an iPhone, keep in mind that most Android devices have slots for removable microSD memory cards—meaning you’re free to expand your storage should the need arise. You can find microSD card up to 32 GB in size online for about $60, while 16 GB cards will run you $25 or so each.