it could be either or both. sorry not much help. aubretia is certainly more of a violet purple, though some of the campanulas can also be a deep blue/violet.

if you buy the plant in a pot you can plant it any time.

I have Campanula that has seeded itself in several places on our wall,Kidsgran..and it was never planted there ,but I have it in the border ,so it found it's own way ...I have seen Aubretia on walls,and bankings,but usually it seems to have been planted in soil,and just trails over..which makes it seem as though it is growing from the wall...and it needs trimming back now and then, after flowering..
I don't trim the Campanulas, at all,and just wait till the flowering stems have died off completely,when they have turned white ,like thin sticks..and they pull out easily,leaving the foliage intact ,to spread the next year..evergreen too,so something to look at in winter..so your daughter has a choice of either,whichever would be suitable for her wall....I did try shoving some of the Campanula in little holes in the wall,but they never took,as no soil as such, to get rooted into..so mother nature did it with wind blown seed instead..hope this helps..

Thank you both very helpful my daughter as a fence above a retaining wall so she is thinking of plant something under the fence to trail over the wall so I will pass on the advice to her , it's funny she has 3 boys who have kept her busy over the years and are now 22, 19, & 15 so not so many activities to be driven to as two drive them selves so she as time a last for her garden and keeps asking me questions many I can answer as I have always gardened in my own way , but since joining Goy can pass on so much more to her thanks to you knowledgeable people :o)

That sounds ideal for either,Kidsgran,and glad your daughter now has some of her own space :o)
Aubretia is looking good at this time of year,and the Campanula is just starting ..maybe a bit of both,to extend the flowering season ? ..just a little tip,there are a few different ones in the Campanula variety,so she needs to get the trailing one,if she decides to go for that..sorry,I don't know the name,but you could google it,or click on C on Goypedia..lots of pictures on there..and mine is on the top row.of the first page, of the one on the wall..along with lots of others ! :o)

As everyone says, Aubretia for early to late spring, Campanula for May to June - the ones you usually see are Campanula portenschlagiana (syn muralis) or C. carpatica, C. porscharskyana, (not sure that's spelt entirely correctly but its close enough). Both the latter are a bit sprawly and put out long loose growths with flowers on, whereas C. portenschlagiana is a much neater grower.