There are several factors to consider when you're choosing a paid VPN.
1. Does the plan have servers in every country and region you need? Having more than one server in a country can help spread the load, but doesn't guarantee improved performance, so don't assume a plan with 500 servers will automatically beat another with 100.
2. Check the number of simultaneous connections supported. Typically, this is 3-5, which allows you to have a PC, mobile and tablet connected at the same time. But beware, many companies say this is for a single user only, and they all have fair usage policies to prevent people hogging resources. If you let the entire family download and stream videos separately then you'll run into trouble.
3. Some providers list the connection protocols they use. OpenVPN and IKeV2 are good choices, fast and secure. You might see SSTP and the older PPTP, as well as protocol options (TCP or UDP for OpenVPN). You don't need to understand the low-level details, but having the extra choice can help the service make faster and/or more reliable connections.
4. All VPN companies say they don't log whatever you're doing online, but inevitably they collect a little data. Some services record the day your account logged on, the amount of data you used, and delete anything else when the session closes. Others add items like your incoming IP address and the server you used, and keep the data for months, even years. If you're concerned, check the Privacy Policy and Terms of Service to find out more.
5. It's important to consider the client, the software which handles your connections. These all have a list of servers and a Connect/ Disconnect button, but could you use more? Some clients display server load and ping time in the interface, helping you choose the right server. Regular users might appreciate a "Favourites" system to save and recall specific servers. If you know what you're doing, having access to low-level network settings will help you tune the whole system.
6. Finally, there's the price. Beware of apparently cheap deals: these may have restricted features, exclude taxes, be discounted for the first billing period only, and renew automatically, so that apparent one-off £3.99 might become almost £10 next month. Look for a 'Pricing' link, read the small print, and if possible use something like PayPal where it's easy to check and cancel a subscription yourself.
Once you've found what looks like a good VPN candidate, be sure to take it for a trial before you spend any big money. But a short trial can only tell you so much, so once that's expired, pay for a month, run as many tests as you can, then upgrade to a better value plan (usually yearly) if you're still happy.