There is a difference in 1300 & 1800 engine. Difference of 500cc means 500cc more power than 1300cc in same body. For details specs what u get more in Altis and what u dont in GLi can be found on Toyota website. Link given below.
Now let me talk about my experience.
In Local drive altis and GLi given same fuel average. 9/10 KM per liter
On motorway altis and GLi given same fuel average 14.5 KM per liter @ 120 KM speed but if you reduce your speed @ 100 KM then ALTIS average is 15.7 KM and GLi average is 14.8 Km to 15 KM per liter.
Despite of fuel average I found GLi is under power especially in mountain area even you can compare its performance on motorway at salt range area. Altis can easily climb @ 4th gear without any trouble with Ac no gear shift required. But for GLi sometimes you need to go down to 2nd gear to maintain speed.
I visited Shogran in March 10 and my brand new GLi is hardly climb in 1 gear sometimes I need to switch off the AC but in June 10 again I visit to Shogran on ALTIS its easily climb in 2nd gear with AC.
ALTIS is equipped with AIR BAG, Optitron Meter, Reading Lights, Wooden Panels, Alloy rims, Side Skirts, Colored mud flaps & door handles, Door moldings, 2 Extra speakers etc.