I would think a small heater not needing to run that much and the efficiency of the LEDs would come out less. Plus, the LED will be providing you useable light for your plant instead of wasted energy being turned into heat.
You probably wouldn't need a 600W LED for a 2x4. A small 250W desk heater will do just fine in that space. Put some budget in for a temp control for the heater and you should be good to go.
LED lamps will put out some heat and so do their drivers. I use a Honeywell HeatBud in both my tents, a 2.3x2.3 and a 4x4. It manages to keep both sizes at 75-80F. I use an inkbird controller so it is not on constantly. Just an idea!
Buy it from wherever you like, usually $25-35. There is a mechanical on/off switch so it's easy to use with a timer or controller. It also has a I and a II setting with I being 125W and II is the full 250W. Fire safety switch on the bottom so if it falls over it shuts off.
Heat Bud Ceramic Heaters feature a 250 watt output that's ideal for heating personal to small sized areas of space. This unit includes two differing heat settings that allow users to shuffle between varying levels of performance.
www.honeywellstore.com