I was wondering what the practical difference between the two in regards to climbing and descending altitude?

In climb, they're pretty similar. Both will fly you at N1 REF to read your MCP altitude as soon as possible. One difference is that if there are climb restrictions, FLCH won't obey them while VNAV will.

 

On descend, VNAV will follow the FMC vertical path (VNAV PATH), while FLCH won't.

Flight level change is  a "Pitch-to-Speed" altitude mode, could also be called a "Speed on Elevator" mode. Basically, it'll try to achieve the Altitude set on the MCP using the IAS selected at the speed window as reference, using pitch.. For example, if it's used during climb, 10000' is set at 250 IAS, It'll pitch the airplane to maintain 250kias using Climb thrust untill 10000' feet, if it's a decent, it'll do the same, but at idle thrust. If your are faster, i.e 280kts, it'll set engines to idle, but first it'll command a shallow descent, so it can lose speed until stable at 250. Remember, FLCH won't take any altitude constraint set in your legs page into consideration, so it's up to the pilots to monitor them. VNAV  is a hybrid mode, it has elements of various modes into it, to satisfy whatever performance/constraint setting the FMC has computed.

To add to Jefferson's good answer, you need FLCH because VNAV allows you to climb, cruise and descend in that order and only once. So, if you need to cruise lower for ATC this is the only way to do it... other examples abound.

FL CH is an all-out climb or descent mode. It will use either THRUST LIMIT to climb or IDLE to descend. It doesn't care about vertical profile, speed or altitude constraints, except for what is set in the MCP.

 

FL CH is not as dumb as it used to be - it now cares if the altitude change is small or large, and uses a reduced vertical speed for smaller changes, to increase passenger comfort.

 

VNAV is far more complex. It cares about the vertical profile through space (geometric path), as well as speed and altitude constraints. It is the LNAV of vertical modes.

 

Best regards,

Robin.