Tax on Advance for sale of goods has been stop. On service it is still chargeable and Have to prepare advance voucher as per accounts rules. So it is better if you show it in gstr1.
Gstr 3b presentation : Advance has to be in unleaded in total supply colum as formula of taxable supply provided below 3b includes advance. Gstr 1 presentation : 1. Add to advance coloum if advance is received and sales is not there in same month. 2. And when sales happen Show sales as per b2b or B2C and show the advance adjusted in coloum provided for advance adjustment.
As regards to not showing in 3b and showing in gstr 1 You will incure additional tax liability and interest there on.