I guess you could break it up into features and performance.
Features would include:
- 10 Gb interfaces
- redundant power supplies
- expandability/scalability/upgradeability
If you don't need 10Gb interfaces and your setup isn't likely to need to change in the future, then there isn't much difference between an 8610 and a stack of 5510s. Otherwise, assuming this is for a larger network, the 8610 would be the smarter bet.
One issue you shouldn't overlook is support and maintenance - with untrained staff, the 5510s are easily supported, whereas the 8610 would not be so easily supported.