When researching an engagement ring it's important to take into consideration a number of very critical and fundamental things. The size of the diamond is of course of importance as it will dictate price, as well you will have to look at the cut of the stone, the style of band you wish buy, the clarity of the stone, etc. These are things an experienced Jeweler would be able to assist you with.
Other factors like the table will also help with the appearance of the stone and how big the stone is perceived to be. Having a stone with a larger table can make a smaller stone appear larger and could give more value to the stone to eye of the beholder.