Do you or your company use AutoCAD Land Desktop for your design? If so, StormCAD is better because it can interface with the "pipes" modules of the CivilDesign package included with LD3/LD2004/LD2006.
The "pipes" module allows you to design your storm sewer in a stick-figure format, and has built in tools to do most of the profile drafting, and puts all structure and pipe data into an MS Access database. This database can then be loaded into StormCAD to build most of the analysis model.
On the other hand, Hydraflow StormSewers is not a bad program and works just fine. It just doesn't integrate with Land Desktop.
The original post specified the use of TR-55 methodology. As far as I know, neither Hydraflow Storm Sewers nor Stormcad has this capability integrated (one can manually put in peak flows.) They are based on Rational method.
The use of TR55 for peak storm sewer design is sort of cumbersome. TR 55 and TR 20 are more tools to check the response of a watershed to changes in the watershed. There is a chart which relates CN's to C-Factors. I usually compute the peaks using Win TR 55 and then place them into a storm sewer spread sheet. You might find on drainage areas less than 0.50 acres no flow is computed. You have to use some engineering judgement for these areas.
The Rational Method will do fine.
Use the 5-minute rainfall duration -- this provides 500-1000 acres of raincloud size. Average rainfall intensity for the cloud is given in IDF curves.
If your site is relatively flat assume the best you can do is a flow velocity of 3-4 ft/sec.
Do not go above 6.0 ft/sec -- you will probably be surcharging manholes & CB's.
Stormshed software can do both, but can be buggy to run. (Make lots of backups as you go) IMO it is way too expensive for the product ($2500 USD), but you have to consider how long will it would take to couple TR-55 hydrology with a separate hydraulics program, maybe it's worth the cost for you. Note that the hydraulics is slightly black box, review the manual before you buy if possible.
EPA SWMM could do both, and is !FREE!, but has a steep learning & setup curve.
HEC-HMS is widely recognized by environmental agencies like the DEP for permitting intents. If you still need the TR-55 method, HMS will accomodate. It is listed in the basin transformation methods as SCS method. This is the exact same method, just a different name. Hydraflow does TR-55 as well, but HMS is free for anyone to download for the HEC website. Plus, the software is probably the most widely used and respected.
