What's the difference, if you know what you doing .... I see no difference. But in your case you must be sure where this field is created in which model. For example if as you said sale.order.form, field you creating must be related to the sale_order. If not you will not be able to show it.
So PY don't say it's wrong you can use either custom module or create fields inside odoo using GUI :)

If you can restart the service, that means you have access to the server files. So why not create a custom module, instead of creating fields through the gui ? There you go

https://www.odoo.com/documentation/8.0/howtos/backend.html#build-an-odoo-module