I don't have any experience with RP125, so I cannot compare.
I bought GS150 in mid 2008, I haven't used it much(only over 20,000kms), as I don't have daily usage and mostly use it for out of town trips and I sometimes don't even start it for a couple of months. It stopped for the first time after four years due to an intermittent CDI unit and hasn't stopped since replacing it, so its reliable.
I haven't changed anything, except the rear view mirrors, I now use Suzuki Sprinter's original mirrors with position fixing screw. One of the front shock tubes was replaced under warranty, within 2 or 3 months of purchase.
For me tuning means a 250kms round trip to Rawalpindi. I only take it to M R Suzuki, the Suzuki showroom/service centre. I only use original parts, some call these parts expensive(imported parts generally are a bit expensive), but people don't compare these with imported/genuine parts for other brands, which are just as expensive. I had to replace the throttle cable and front brake lever and these were cheap. The CDI unit was expensive, almost Rs.4,000.
Since you will be using it for daily commute, I suggest you borrow it from someone and ride it on your regular daily route before making the decision. GS150 is not meant to zig-zag in crowded city traffic, its handling is different from 70cc to 125cc bikes.
The bike normally gives between 40 to 42 kms/litre for city riding and between 48 to 53 kms/litre on GT Road(Abbottabad - Lahore), depending on speed.
The rear tyre still has a little life left, at least for road trips. I never had any problem with road grip, the only two times it felt like skidding were immediately after service/brake pad replacement. I have seen most people over-inflating the tyres, losing traction and wearing off the treads within 5,000 to 8,000 kms.
I avoid over-speeding, so the drum brakes work just fine. Remember CD175, CD185, CD200 with top speeds reaching 140 kph and only drum brakes.