Here is a really difficult question to answer - do you need a smartphone 'phablet' with a 5.5 inch screen or a tablet with a 7 inch screen? Or is it better to just have a regular sized phone (approximately 4.5 inch screen) and a 10 inch tablet?
Don't jump to say the smartphone because it's better VFM to have a two-in-one device. You may find that it is neither fish nor fowl, and does not satisfy you as either a communication or a browsing device.
These days I get a lot of queries about the best screen size for a tablet. The answer of course, is that it depends on the purpose you are going to use the device for.
Smaller tabs like the Kindle Fire, iPad Mini and Google Nexus 7 are appealing because they are lower priced than their bigger siblings. However, a 7 incher may not give you the best usage value if it does not fit your purpose of use.
Here is a handy guide
4.5 inch/ standard sized smartphone
This is more than enough for daily short burst of use - email, SMS, casual browsing and gaming. I find it a strain to use it for prolonged periods in the airport waiting lounge, or in a moving vehicle. As you get older, you will probably experience a strain on your eyes from prolonged usage of a smartphone screen. In which case, it becomes time to supplement it with a tablet.
5.5 inch smartphone (eg. Galaxy Note)
If you talk less on the phone but do more of browsing, doodling and typing, and do not wish to carry a tablet or laptop everywhere, this is for you. Also, you should have access to a power outlet for charging. If you buy this device, you may find that an additional tablet is redundant. In that sense, it does save you money. Do note, the first thing I said is, use this if you talk less on the phone. This is not a device you can hold up to your ear easily, plus calls will drain the short lived battery really fast.
7 inch tablet (iPad Mini, Nexus 7 etc.)
This is a tricky one. Buy a 7 inch tablet if your purpose of use is mostly reading of e-books, heavy email usage BUT light browsing, and you also travel a lot. I thought this through carefully. If you travel a lot, even an iPad starts feeling heavy soon! A 7 inch tablet is way lighter, fits more easily in a handbag or large pocket and gives you instant access to a screen which is larger than your phone. It's a perfect size for e-book reading, especially on the flight, and it's way easier to compose mails on it compared to a smartphone. However, I still favor the browsing experience on a 10 inch tablet more than this.
10 inch tablet
Shell out the extra bucks for the full monty 10 inch experience if you intend to use the tablet regularly as a replacement for a netbook or PC - to view movies, browse websites, read,edit or create work related documents and excel sheets. For any sustained usage over a longer period, the bigger screen begins to make sense and its something your eyes will thank you for!
As you can see, additional choices can make life more confusing, but at the same time, enable you to spend exactly the right sum to get a device of your choice!