Hi Mark,
You're looking at two very good rifles there. Lightweight, accurate, excellent triggers and with typical precision German build.
But I'd also suggest that you throw the HW95 and the Diana 31/34 variants into the mix.
The main deciding factor should be how they feel to you and which you think suits you best as regards handling, weight, dimensions etc. All the rifles will be excellent.
I own two HW99s and two 95s and love them all. You may be aware of some "issues" regarding the cocking lever to cylinder wear on the 99? They don't all do it. And it's fixable. And to not buy one because of this would mean that you're missing out on a superb little rifle with a beautifully refined firing cycle, contradicting the old notion that you can't have a full power, lightweight rifle that is so nice to shoot and so easy to achieve real world accuracy with. However, if the scare stories, er, scare you, the 95's a superb rifle.
I have had only brief experience of the Diana 280, having had a few goes with them. And loved them! Lovely handling and I really like the way the action feels "low set" and "flat" into the stock.
I have zero first hand experience of the 31/34 Dianas, but many owners are extremely pleased with them and I see no reason to predict that they would be nothing other than excellent.
One criticism you do seem to hear of the Dianas is that they are very dry from the factory. Ideally a strip, clean and re-lube would see to this, but if you didn't want to strip initially, some careful application of moly grease to the spring coils, rear of piston and cocking slot should be fine.
Now that the Dianas are available at more realistic prices over here, I'd predict they'll become more popular.
Hope this helps a little. Try and handle as many of these as you can before deciding. Better still if you can have a go with them, ideally at a club / longer range.