Sometimes it's difficult to decide if you should spend money on the pro or full version of an app. The free version may be "good enough," but sometimes you can get rid of annoying ads or gain extra features for just a few bucks. In many cases, the way to decide on buying an app will differ for each app and each person. Here are the points I consider, along with some from an article on Lifehacker, before I buy an app:
Read the list of benefits
What sort of new features will you get once you purchase the app? When I'm considering the Lite vs Pro version of an app, I look for a list of upgraded benefits in the description area. When an app is only 99 cents, I usually go for it--even if no ads is the only benefit. This seems to be the magic price for a lot of apps. If the app costs more than that but opens up an entire new area of features, I think that's another good reason to buy it--provided I can be see myself using them. On that note, you might be interested to check out Rafe Needleman's post about why we're not paying enough for apps.
Browse the buyer feedback
Comments should be treated as a privilege, not filled with spam and trolls. Unfortunately, that's not always the case so I find it's good to use some common sense when navigating this area. Some users will be totally off-base about what an app is able to do, while others will have devices that are listed as unsupported. And you guessed it, they both complain in the comments.
By checking out the developer description about what the app does and what it supports, you can learn which comments to ignore and spend your time reading what matters: actual feedback. Don't forget to check out other apps the developer created. This may not make the case for just one app, but it can help give you an overall feel of how happy their users are.
Buy it, try it
Perhaps this is the most obvious of the choices, but it comes with some caveats. For Android users: Google Play (formerly the Android Market) shortened the return policy window on paid apps to 15 minutes a long time ago. So if you think you can grab the app, use it for about 12-13 minutes and then make a decision on whether you want to keep it...go for it!
Just remember that some apps are catered toward situational usage (like a reliable Twitter client that notifies you of updates/mentions), and those 12-13 minutes might not do. Additionally, paid apps paired with PC or Mac counterparts that need to be configured may gobble up all of your return period. For iOS, you have to request a refund from the App Store, instead of doing it on your own. Overall, I'd say this option is best recommended for wallpaper, soundboards, widgets, and the like.
Search for reviews
If you're a fan of second opinions like I am, you'll probably want to see what other users across the Interwebs think of said app. It's very likely that you will see people who review technology for a living (like some of the CNET staff) discussing an app that has a price tag, especially if it's a great or terrible buy.
However, I also encourage you to read what other blogs and individual users think about the app. After all, it's no secret that when you're always looking at apps they can start to look similar or fail to surprise, causing reviewers to sometimes be more critical. While this isn't always a bad thing--since it helps you pick the best of the best--it's still OK to like or want an app that doesn't interest the people writing about it. One note, though: if all you can find are bad reviews, you might want to think twice before you waste your money.
Ask friends or social media for opinions
Oh, if there is one thing that your friends or the Internet will be happy to share for free every day, it's definitely their opinion. With no shortage of these, I often find myself asking, "Hey, I know you use App X, but did you buy the upgraded version," followed with "why?" I probably won't go on the opinion of just one friend, but it's a good starting point for trustworthy feedback.
Next I'll turn to social media like Facebook and Twitter. Most of my Facebook friends are people I know in real life or work with online, so I can trust most of them to stand behind the opinions they share. Twitter can be a mixed bag of results. I don't know everyone following me, but I do find most of their input to be extremely helpful. Sometimes I'll get an answer to a question I didn't ask, while other times I'll get actual feedback and even recommendations for apps that are better at doing what I want.
Ah, the magic and mystery of the Internet.
Wait for a sale
The rebrand of the Android Market, now called Google Play, opened the door to some amazing 25-cent sales on featured apps. Amazon has its own Android app market, and often runs promotions on paid apps--either for a reduced price or even free. The same can be said about iOS apps, as sites like FreeAppADay and AppShopper will run specials on apps.
How do you decide whether or not to purchase an app? Share your methods or thoughts in the comments!