Hollyhocks tend to grow like a biennial, flowering in the second season from seed, although they can survive to flower more several more years. They flower over a long period. Sometimes they will flower in the first year. Young plants bought now should flower this year.

Rust might not be a problem when you first try hollyhocks, but it will turn up sooner or later and can defoliate plants by the end of the season. Once the rust sets in you can control it a bit by religiously getting rid of all the hollyhock stems and leaves before winter.

Delphiniums have at least two flowering sessions in a growing season - spring and autumn. They produce, then die back to a miserable little knot of leaves. If you side dress them with good mulch and are not too lavish with the water, they'll come away again.

The old one I have up the garden has been going since 1999 through some appalling seasons. I bought it a friend a couple of years ago and it performs similarly.

Last year I bought one at the end of the season. It was solidly pot-bound. I repotted it after some root surgery - and it promptly fell prey to a snail attack. Muttering dark things about idle blackbirds I moved it in its pot to a drier place where it recovered. I've since planted it out. It hasn't flowered yet, but neither have the snails come by it.

I suspect that the Slimy Brigade goes for Delphs that are over-watered, and somewhat down on their luck. Good compost, applied as a mulch - and a gang of blackbirds...

The slimy ones also go for baby Delphiniums :( I lost an entire crop to the little swines...no Delphiniums for me last year! This year I'm armed and dangerous!

Anyway...back to the question...

Hollyhocks and delphinums each have their own problems, neither is necessarily easier than the other, or harder than the other. On the whole Hollyhocks flower longer and are therefore more flower value for your money, That said, eventually you may get rust - which is dealable with as Shrubs has pointed out. Delphinums - once they're no longer a tasty snackette for a slimy thug - are less prone to problems, but their short flowering life and large ungainly remains make them a problem for some people. I believe I read that Gertrude Jekyll advocated growing a late flowering clemetis over the delphinum remains - thus you still get height to the garden and the delphinium carcasses are nicely covered.

Why don't you grow both for a couple of years and see which you prefer? Personally I don't think you can beat the blue of delphiniums and that alone for me makes them preferable to hollyhocks. Obviously your mileage may vary!

Alcina

very useful replies from all. I think I'll go for hollyhocks but I will grow one blue delphinium and see about getting a clematis to grow over it. Great idea.
As alcina suggests the short flowering period and the unsightly remains are a problem for delphs I think

Once you get the delphiniums up past the 4 or so inch stage the molluscs don't seem to bother them. So if you can fight them off for the first crucial few inches you may not have any probs after that. It also depends on how wet/dry that section of the Spring is. I look forward to my delphiniums (which I raised from seed about 5 years ago) every year and don't mind a shortish flowering period. After all the flowers are over I cut the stems back and when the foliage is really getting out of hand I cut it back to the ground, every bit, feed them, and plant takeover plants nearby - usually in late July. I do this to oriental poppies, too, and to geraniums. Usually a small rosette of replacement foliage then emerges. Not sure how they like it, but as long as they reappear every year, and they have so far, I'm happy. My garden is quite small and totally overplanted so I have no option but to treat them like this!
My hollyhocks get a bit of rust but also reappear without much encouragement. They self-seed easily so I'd go for the seed option. Dozens more plants and the colours of your choosing.
I trust you're taking out a second mortgage if you're planning to buy them from Homebase??

If you want a hollyhock that blooms the first year and doesn't grow to an ungainly height (it reaches 2 1/2 feet), try Queeny Purple, which was an All American Selection of 2004. I assume it's available in the U.K.

Here is a link that might be useful:

I grow hollyhocks and find that they got rust here in the first year of growing them. I've since discovered that rust over winters on the spurge weeds and am pulling out every spurge plant I can see in the hopes of controlling the situation this year.

If it doesn't work then I'll keep to growing Sidalceas instead. Although they only look like teeny hollyhocks they're still very pretty.