If you are looking to build a home layout, sectional or portable is probably preferable to modular since you won't be changing things around much so you don't need the interchangeability.
If you are interrested in building a module in order to use it at a modular club meet, you should probably check with a local modular club. The ho modular club I joined was in existance as a modular club before the advent of n-trak, and way before the NMRA set modular standards. In fact the NMRA asked for copies of our club standards before adopting standards for modular clubs. NMRA modules will work with our club modules, but they would need special joiner tracks because we use a 2 inch set back that requires a 4 inch joiner track while NMRA uses a 4.5 inch set back to use a standard Atlas 9 inch sectional track for a joiner. Therefore a module built to NMRA standards needs a 6.5 inch joiner track to mate up with our set up. We use 4 inch joiner tracks for 2 reasons.
1. all but very small diesels and small steam engines will bridge a 4 inch joiner, picking up power on either side of the joiner track. Since all track sections have drop wires soldered to them, but joiner tracks rely on rail joiners to cqarry power, a locomotive that doesn't bridge the joiner track could stall on the joiner track if the rail joiners make a bad connection. With the 9 inch snap track joiner, a gp9 could stall out on the joiner if the rail joiners loosen up.
#2 with a 2 inch set back we can put a 36 inch minimum radius on a 4 foot x 4 foot corner module with easments. The NMRA standard modules are locked in to a maximum of 30 inches on a 4 x 4.
Finally, if there is no modular club in your area, but you would lie to build a module in order to hook up to other modules at a national or regional NMRA meet, then build the module to NMRA standards since that is as close to a universal standard as you will find.
Here is a link to the home page for the Orange County Module Railroaders:
Orange County Module Railroaders- So. California HO scale RR
The following is a link to our module standards:
OCMR Module Standards