To give the correct advice we need to know the size of your enclosure, as the exact UV bulb you select will depend on how high it is mounted above the basking area. The thing to remember is that it is UV Index that is important; many bulbs can achieve the same index at different distances.
For Corn snakes it is generally agreed a UV Index of about 1.0 to 1.5 is adequate (I have personally given mine the opportunity to bask at higher UV Indices and they will, but a range of around 1 to 1.5 is adequate).
Depending on distance you can achieve the same UVI from a T8 fluorescent or a T5 Shade Dweller as you would from a higher percentage bulb. At 24" distance from an Arcadia 6% t5 fluorescent, you will get between 1.0 and 1.5, the advantage of this bulb though is that they also last a lot longer (I have some that still emit UV after two years, and they are guaranteed to last at least one year; Frances Baines has reported one that still emits enough UV after four years). All of those choices can work depending on distance.
Personally, I would go for a 6% Arcadia t5 in a 24" high viv, but you have to be careful not to allow the snake to get too close to it; at 12" distance the UVI goes up to around 4 or even more after the bulb has been burnt in. I am not saying this is 'too much' but it is certainly too much if the snake cannot get away from it. If you are going to put branches under it (and I suggest you do) I would also recommend ensuring they are not closer than 12-18" from the light, and that the light does not extend more than half the length of the terrarium.
The way I get around this is to have the light up against the front of the top of the viv. This means that you not only have the distance between the ceiling and the floor of the viv to play with, you also have the distance to the back wall of the viv too; it is now easy to stick some higher branches without the UV exposure becoming too high.
