iPhones run spreadsheet apps well. Deciding which spreadsheet app is best will depend on what you need to accomplish. Numbers, from Apple, is made for the iPhone and will run efficiently, consuming little battery or data. Microsoft's Excel may be more familiar to many or most people with any desire to save a spreadsheet on an iPhone. So which spreadsheet app is best for you? The answer depends on these things: whether you need to share data with other people, how frustrated you will be learning a new spreadsheet and what functions you need to use.
Microsoft Excel comes in a free version for iPhone under the name Microsoft Mobile Office. This edition lets you view Excel spreadsheets that were setup elsewhere and mailed to you using your handheld device, but it's not useful for tracking data as the free version does not allow you to save updates you make. To save files you'll need to get a Microsoft Office subscription license that costs about $100 each year for up 5 to devices.
Numbers, the Apple answer to Excel, costs about $10 through the Apple app store. That's a one-time charge and it's good on all the Apple devices using a single Apple ID. So if you buy Numbers for use on your iPad, you can automatically use it on your iPhone, too. Excel users may find Numbers difficult at first. The basic spreadsheet functionality is present, though not all the Excel functions. With perseverance users may find they can get basic spreadsheet work done using Numbers, even on a small iPhone screen.
So which is best? If you want to read spreadsheets from time to time on your phone, the free version of Microsoft Office is all you need. If you want to collect and track data on the fly get Numbers and learn to use it. Numbers will let you save a spreadsheet as an Excel sheet and email it to others, but it won't do every Excel function, and it doesn't format in the same manner as Excel. If you want to be able to work on Excel sheets and share them with other people, using your phone or tablet, determine whether the Office subscription you use at work or school can cover use on your mobile devices; if not, you will probably need to spring for the $100 annual subscription for Microsoft Office.
There are other hardware options worth considering if you need to use Excel spreadsheets outside the office. The Windows Phone and the Microsoft Surface tablet both are designed with the Microsoft Office suite of products, including Microsoft Excel, in mind. The Microsoft Surface tablet is larger than most tablets but smaller than a traditional laptop, and comes with a full-function, detachable keyboard. The Windows phones are manufactured by cell phone veterans Nokia and HTC. Though not as popular as iPhone or Android phones, Windows phones work well. They come in the same sizes and shapes as most Android smartphones. See photos in the attached slideshow for examples of the Surface and the Windows Phone.