I'd agree with Gripper. I have both hard and soft tops, and the difference in water tightness is un-noticeable in practice. I use the hard top in winter and soft top in summer, and the only problem I've had with it was entirely self inflicted... I folded and stored the soft top before it had completely dried out, with the result that when I unpacked it in the spring, it was a bit mildewey, and started to leak. Doh!! Quickly fixed with Thompsons Water Seal though, although as Fergie suggested, you need more than you think. From memory I used the whole of a 4 litre can!
Security can be an issue if you keep stuff in the back. However, I owned a completely knackered old heap of a Series IIa in Wandsworth in London for about 2 years. That had a soft top, and I drove it and parked it all over London with the back rolled up, without any problems at all, (except for someone chucking a fag-butt on the roof one night... probably to see if it caught fire... it didn't). Mind you, that old heap didn't have any door locks either, proper farmer spec, and you could turn on the ignition with your thumb-nail!!
And as others have commented, I wouldn't want to roll a soft top, but then I wouldn't fancy doing that in a hard top. Neither offer much protection, but I don't plan to roll mine, haven't come close to it yet, and only do 3,000 miles a year in it, so it's not an issue. But if you do serious off-road work, or do lots of miles in one, then fit a roll cage.
I would go for it! Remember that if you are expecting a few good sunny days, you can take the top off completely, take off the door tops, and fold down the screen. Proper wind-in-the-hair motoring, fantastic fun (but remember that bluebottles, horse flies and bumble bees seriously sting at anything over 40mph!!).
Hope this helps,
Effortless.