VNAV SPD climb in ECON mode may feel like a smoother ride as it will use FMC information as well as inputs from air data computers etc to provide an optimum climb speed at rated climb power. This generally works out at about 2000-2500fpm up to about FL300.
LVL CHG is a good mode to use if required to make a more drastic change in altitude due ATC requirements (or stuffing up a descent!!) as it will fly the MCP-commanded speed at rated thrust to climb, or maintain the MCP speed with the thrust levers closed to descend.
Selecting LVL CHG during a VNAV SPD climb and selecting a lower speed in the MCP window (eg selecting 250kt during an optimal 295kt climb) will temporarily result in a large change in attitude as the autopliot tries to reach the commanded speed at normal climb power. Rates of climb can periodically go off the scale (ie >6000fpm) in extreme cases, therefore the climb will appear to be less smooth....
Otherwise I can't say offhand if it is any smoother over longer periods as the automatics are programmed to make pitch and roll changes within a specified (very low) G-tolerance.