First off, do not confuse a digging spade with a shovel. A shovel has curved up edges and is used for what it says - shovelling. They are usually seen being used by road workers and used for moving a lot of bulk from one place to another and is not suitable for digging in the garden.
A digging spade has a flatter blade and does not curve up at the edges.
A good spade is essential for digging deep beds. It is also indispensable for many other chores around the garden: digging straight-sided holes for trees, shrubs, and perennials, slicing and tamping grass sod, prying up rocks and stones, transplanting, cutting heavy weeds, edging lawns and garden beds. It should keep a keen edge when sharpened which makes digging much less of a chore. I have seen old spades that have lasted so long and been filed for so many years that the blade is half its original size.
Buy something like a Bulldog brand, I have never heard of a blade chipping, bending, breaking or splitting as is the case with common sheet-metal, tang-and-ferrule, or welded tools. There are no welds on the neck to break or weaken. These spades are made with treads, a narrow steel plate on top of the blade so that the foot can leverage hard, compacted soil. Most spades comes in two lengths, the 40" length for people under six feet tall and the 44" for those over six feet tall and 2 sizes - The ordinary full size spade and the smaller lighter Border spade. Border spades are particularly suitable for ladies use and in smaller spaces.
Alternatively, invest in a stainless steel spade, especially if you have heavy clay soil, as the clay does not stick to stainless steel as much as ordinary carbon steel.
The handles are more a matter of preference than anything. Wooden handles are more traditional and are usually replaceable if needed, but most modern spades tend to have steel handles.
The top shape of the handle is just a matter of preference also, some people prefer a 'D' shaped handle, some a 'Y' shaped and others swear by a 'T' handle. It is best to try it out when purchasing the tool to find your own preference.
There is a lot being said at the moment for the American type spade. 
This has a more rounded blade a a very long, straight handle with no sort of bar at the top. They are said to be a lot easier to use, particularly for people with back problems, than the traditional English type spade. The only problem is actually obtaining one, at the moment there do not seem to be many suppliers in the UK.
Buy a good quality carbon or stainless steel fork. The cheaper pressed steel forks have a tendency to bend easily. A fork with square profile tines is preferable to one with thin sharp tines as their broader tines allow you to lift and turn soil much as you would with a spade. It is also ideal for harvesting potatoes, leeks, beets and other root crops, since the flatter, blunt tines do not pierce the vegetables. This is the more common fork and preferred in areas where the soil is heavy.
As with spades, forks come in two sizes and lengths. There are also different types of forks designed for special uses such as Potato forks - these have broad flat tines designed not to damage potatoes, Manure forks that have longer, very thin tines and Pitch forks with only two long curves tines.
