I've had a 1.8 for a couple of years now, and just bought a 1.6 for my donor.
There's not a lot in it, but the 1.8 does feel a tad quicker.
Either should be a whole lot of fun in the roadster though
If you do go 1.6 I'd avoid the early shortnose crank engine (up to about 1991).
The 1.8 diffs might be heavier, but they're also a lot tougher. The 1.6 diffs are a bit weak and break if abused - particularly if turbo'd.
You can retrofit the 1.8 diff onto the 1.6, but they go for silly money, and you have to change the driveshafts and propshaft whilst you're at it.