Assuming no unusual circumstance, always lap splice, and to the extent it makes sense, class B tension lap splices everywhere. You should not lap splice large compression bars.
For certain conditions, you may want to consider mechanical splices: very large bars, heavy congestion, perimeter beams with blast/continuity considerations. Mechanical splices are quick an inexpensive, but do require more time and cost than laps.
Welds should not be used unless you can be sure all of the requirements will be met, and the welding will be competently done in a manner equivalent to shop welds. Field welds should only be attempted on ASTM A706 reinforcing steel. Field welding will slow work and require new trades on the job (welders, weld inspectors).
John Turner CSP PE
CRSI Greater Southwestern Regional Manager