Designing it yourself is likely to save you a lot of money, and in addition you can really put your own personal stamp on it if you feel that you have the relevant creativity to do the job well.
Another key factor will be customer satisfaction. When it comes to any design of the website you really need to put yourself in the customer's shoes. It seems like common sense to list this as a key point but many people and companies will be rushing through the different steps on the design process and will often fail to engage with the idea of what the customer might actually want to be viewing on the website.
Another key point will be the budget that you are willing to set out. If you are going to be using a professional design service in order to get your site up and running, then you are going to be spending a bit of money. As such it is important that you set up a budget for doing this so that you do not overrun this too much. If you choose to design and set up the website yourself, then you should be able to do this at a limited cost.
It is also going to be very important for you to really consider the content of the site. You need to start building a list of what you are going to be adding onto the website, so that you can refer to this as you go through your planning and organising stages. A good idea is to keep a separate file on your computer which will list all of the different sections to the website that you want to add on. This will all really come down to what sort of website your running, and so it is important that you consider exactly what you are offering when you are making this list.
These are just some of the main points that you need to really consider when you are designing a website.