Which one should I buy? Lenovo a6000 or Microsoft lumia 535?

It's upon your requirement
Lumia 535 will be receiving Win 10 {){I'm using a lumia 730 it's specs are low on paper but when you''ll take it in your hand and use it daily you will start considering low budget android's bad }

Lenovo a6000 & A6000 plus both have good specs on paper but they are not made for long time use. As they tend to get slow overtime.

I'm saying this because currently there's no scope of android coming to PCs whereas Win 10 will be coming to all PCs running genuine win 7 &win 8. You can easily sync games, save your files to onedrive etc in Win10 but you'll be devoid of these features in android phone.

If you talk about the limited apps in windows, remember all apps in windows are extremely good.{Android may have more apps but when it comes to usabilty both windows and android are good}
You will also be getting cortana which is far better than google now.(I have compared it)

I have also used a Moto G before getting the 730 believe me android phone like moto g & lenovo a6000 etc have limited capacity for memory i.e 16 or 8 gb and as it fills up you have no option to move hefty files like
asphalt 8 (full game)to sd card{Some don't have one like moto g 1st gen}. But you can do it with a lumia (it supports sd cards upto "128"gb)

Most of budget oriented android smartphones don't have a good camera which is often found in Lumia's.

Please don't consider this review as biased to windows because android maybe good but when it comes to budget oriented android smartphones they don't stand a chance against windows phones.

Hope it helps

Lumia phones have been known for their value and Nokia-like quality and durability that now comes under Microsoft. Lumia phones also come with Windows Phone, a mobile operating system that has proven itself to be one of the safest and stable mobile operating systems out there.