The definition in the ID attribute would override the class declaration. This is because of the specificity weighing rules used to compute the final CSS attributes that would be applied. You can learn more about these rules here:

http://www.w3.org/TR/CSS21/cascade.html#specificity

Try this code to see the impact of the specificity rules:

<style>
.Class { color: green }
#UniqueID { color: red}
</style>
<div style="color:blue" class="Class" id="UniqueID">This is BLUE because style attributes have the highest specificity</div>
<div class="Class" id="UniqueID">This is RED because an ID attribute has higher specificity than a CLASS attribute</div>

Nik