iincreasinly people are using both
i have several clients, friends and family who have both an 021 and an 027
some do this because they need to go to places that have only one network covereage.
Other have an 027 solely for sms - voice calls to and from an 021

myself, i have an 021 for gprs, and i use for calls to other 021s and landlines(motormouth)
I use an 027 for sms, and calls to 027s

handset are less relevant since people are willing to carry two - they just choose to use the handset with the best price offereing at the time.

most telcos subsidise the handset purchase to recoup the cost from calls - that won't happen unless their pricing is competitive.

I do have a gripe with the cost of mobile data - i think it's priced much too high on both networks.