this is reliable on your netwok and covered area, in other words, this is dependant on inter site distance. In my network, we started with 1800 more uptilted than 900 since 1800 coverage is less, and at that time we had few number of sites. Currently i have both with same tilt settings taking into consideration that inter site distance is approximately 700 meters.
just a quick precision about last post : the attenuation is 2dB more for DCS1800, and don’t forget the GSM900 has usually a higher EIRP (+2dB) than DCS1800, and also, the DCS1800 has an aditional loss of 6dB (free space loss)
so altogether, in indoor, you can consider the GSM900 to provide at least 6+2+2 = 10dB more than DCS1800. It probably closer to 12dB, but it has to be verified.