effect of wheel diameter on line following robot
Summary:


About the only advantage I can immediately see in using the small wheels is that a much lower profile design could be built. (Say as low as 50mm versus 120 mm).  If this is not an advantage to you I'd say that the large wheels were substantially superior for most purposes.
Because (apparently) of different gearbox options both wheels give about the same speed of about 20 kph.  
The small wheels will result in extremely small ground clearance - perhaps  about 10mm in practice.
The Large wheels will have perhaps 4 x as much ground clearance - maybe 40 mm+ depending in how much the body underhands the axle.
The large wheels will be much better for traveling over rough surfaces, small obstacles, carpet, grass etc.




Looks like two wheel size options paired with two gearbox options so speed and torque are about the same in each case.

Speed = RPM x 1m/60s x diameter x Pi

1000 rpm x 1/60 x 11 cm x 3.14 = 575 cm/s = 5.75 m/s =~ 21 km/hour = fast

3000 rpm x 1/60 x 4cm x 3.14 ~= 6.3 m/s ~= 23 kph = fast

So the small wheels give very slightly more speed and so very slightly less torque BUT there is not much in o=it.

More importantly, the larger wheels will be MUCH better at traveling overs rough surfaces and small obstacles and substantially better ground clearance..  

A 4cm dia wheel has a 2cm radius.
 If the robot has 1cm of its structure below the axle centre it will have 1cm ground clearance.

A 11 cm wheel has 5.5 cm radius.
 With the same 1 cm underhang it give 5.5-1 = 4.5 cm ground clearance.
 = 4.5:1 a better than with 4cm wheels.

Unless there were extremely good reasons to do not so I would choose the large wheels. 

As this question will be closed on this site - I am at apptechnz gmail com &lt;- add the missing bits.