A wired network will be faster (100 or 1000Mbps), will be at least as reliable (assuming it's fitted correctly) and will be more secure (an infiltrator must be on-site and have a network cable to get in).
However, it will also probably cost you more to put in a wired network properly (unless you want cables all over the floor, you'll need some cable runs and network points fitting at the least) and doing so may restrict the mobility of your users. If everyone is using laptops and often pickup their machine and wonder across the office to talk to other users, then a wired network will inhibit this current working pattern and may cause issues
The alternative is to buy one or more additional wireless access points, make sure they are on seperate channels, and split your users between them. This will not perform as well as a wired network, but will probably improve your current situation and will probably cost less than a wired network (although you should connect the wireless access points via wired connections anyway).
Switching to a wired network is a really good idea, but if you're going to do so get it done properly, and make sure it's got extra capacity (if you hire new people you don't want to be ripping up the cable runs to add another link).
The bottom line is is fitting a wired network properly within your budget?