These two cities almost have the same weather. Usually, in the early Oct, you could enjoy very comfortable weathere both in Lanzhou and Xian, plenty of sunshine and a little cool night. So, during daytine, a short-sleeves shirt with a thin jacket is ok. During early morning and late night, suggested that you wear warm coat. If you come at other time of Oct, please take your long-sleeves and warm coat since it may have some cold rain. On the other hand, you may feel a little drier in Lanzhou than in Xi'an.