If you had to choose +40 or +45 offset, which one would be safer?

 

I don't think safety is the big concern here. Practicality and fitment are. OEM is +41.5mm

Most aftermarkets have a smaller offset. 18" Tuffants Kimberley are +29. 18" Tuffant Simpson are +25 and 18" EvoCoarse Dark Zero are +20 I believe. The 20" Redbourne Alstons are in a similar ballpark.

I don't think you'd want to go to more positive offset than the OEM pushing the tires inward and perilously close to the suspension components. Not a wild lot of clearance with a stock setup as it is. All the aftermarket rims on here I've seen go the other way with smaller positive offsets. Particularly important if running a 275/55 R20 winter size.

Just my 2c or approximately GB £3.50