Difference between Shimano GRX400 and SRAM Apex
SRAM Apex comes in 1x11, Shimano GRX 400 is 1x10 or 2x10. 

SRAM uses the small shift lever for both up and downshifts. On Shimano system the whole brake lever moves inward and is used for downshifts on the rear and up shifts on the front Personally I prefer the Shimano system as it makes it easier to downshift while braking and give more authority to up shifting at the front. 

SRAM shift levers have a far more positive click to them. SRAM uses DOT fluid in hydraulic brakes whereas Shimano uses mineral oil which I think is a bit cheaper. 

The best thing to do is test ride some bikes and see how you feel with the operation of the shifters. Also pay attention to whether you want a 1x or 2x system. 2x systems give you smaller jumps between gear ratios and a wider overall range.