Use of BMW suspension ....

[DonC]

Don't have a dog in the hunt between SPF and BDR but I can comment on the top issue.
Depends on how the car will be used. If it's just going to be a local cruiser don't worry about it. If you plan on making multi-day trips, definitely.
I make several long trips every season. Two or three 3 day events an one that lasts 6 days. I leave my top and side curtains in the trunk from the start of our driving season. Being from Oregon we get more than our fair share of wet stuff. Don't worry about using the car in the wet. You know it's a high horsepower car with a short wheel base, just drive appropriately.
It takes me about 5 minutes to put the top on or off, not a problem once you've done it a couple of times in the garage for practice. Getting in and out isn't problematic, again, once you've done it a couple of times.
Can't comment on the BDR top but a couple of our local club members have SPF's and their fit, like everything else on the SPF, is first rate. High quality materials and made as an integral part of the car rather than an add-on.
Also helps if you get a few good days with low temps. I've been out for 5 or 6 hour drives with temps in the mid to upper 30's and been perfectly comfortable. Again, adding consideration for the lack of traction with cold tires and pavement.
Back to the top, if you're going to make excursions beyond your local neighborhood I'd say a top is definitely in order. Going into my 9th season after about 55k on the odometer and getting ready for a 6 day run through the Rockies in a couple of months. I got the car to drive and a few creature comforts, while not absolutely necessary, make the experience more positive.
DonC