If you can wait a week there will be some info on the SP4 m3 that isn't available today.
For your stated purpose I would go with the SP4 m3 because it will run cooler, quieter, and hopefully have longer battery life (To Be Determined) in addition to the best Pen support, better cameras, the best screen, Windows Hello logon, & faster SSD. Performance wise there may not be a lot of difference (TBD). Downside is it's currently an unknown quantity, we'll know more soon.
If you went with an SP3: IDK if there's any benefit to buying the SP4 pen for an SP3 other than the changeable tips but it would be an extra expense since both come with a Pen (SP3 w SP3Pen, SP4 w SP4Pen). The SP4 Keyboard with the SP3 would be worthwhile and lists for the same price, you have to buy it separately either way. Certainly an SP3 i5 would do what you need, as should the m3, but today it's a known quantity.