I currently use a SP1 and an XPS 15 9530.
Am doing a refresh, so will get the new XPS 15 with 32 GB of RAM.
So the question remains which SP4 to get? The i7/16GB/256 GB or an 15/8 GB/256 GB. No i5/16 GB on offer in the yUK unfortunately.
So what should I get?
MOFO is exactly right, what you would use it for is the key. The SP4 i5 is nearly as powerful CPU wise as the SP3 i7 and is actually more powerful than the SP3 i7 GPU wise except for 2D Graphics. Haven't seen an i7-6650U yet anywhere to know what it's performance is... should be up there.
Based on what you stated though, you will have the XPS for heavy lifting, unless you needed substantial power for something on the Surface I would just go with the SP4 i5 which is no slouch either. If you need 16GB I believe the i7 is the only option.