Do you have a photo of the space or can you tell us what the goals are? I think the effect may be fairly different based on photos of the whole plant. Hopefully, someone from your zone will weigh in on this.

Well, one is a vine/climber (Ramona) and the other is a tender shrub or small tree (Tibouchina or princess flower). Other than flower shape and/or color, they really are not very similar at all. The Tibouchina has an ability to produce flowers over a very extended season in your climate (maybe even year round) while the clematis will be limited to just a summer flowering period.