I've never shot the HW70, but I have shot a few Tempests. Both .177" and .22". Have to say I find the larger bore to be a more satisfying plinker. Though the pellets move more slowly, the whack of a .22" wadcutter into an aluminum can is just so much more violent than with a .177" pellet. If I were worried about longer range shooting with a Tempest I'd go with a .177" as the trajectory would be flatter, easier to estimate holdover out to maybe 30 yards. But these are not precision airguns. Though I've read the odd report from shooters (especially in the UK) who claim to shoot sub-1" groups at 10 metres, the best I've been able to manage was about 1.5" on a particularly good day. I've shot over 550/600 in several air pistol competitions, using a Pardini K12, so while no master, I am a fairly good shot with a pistol in one hand. But with the Tempest my average is closer to 2" groups at that distance. The thing just jumps too weirdly, and I've never been able to master whatever it is one needs to do in order to tame that forward-and-down motion which happens while the pellet is still in the bore. I've filled it at 300 frames per second and analyzed the frames, so yes, I have proof. It's a weird kind of recoil. So couple of British fans have told me the correct manner with which to shoot a Tempest is a two-handed trip with a medium strong hold. I can't bring myself to enjoy that style, but perhaps they're right.
Anyway, I really like the Webley pistols, but more for their aesthetic. I especially enjoy the look and feel of the older models. Excellent bluing, really nice design work, a rock solid feel with all that cast iron. My favourite inaccurate plinker of the bunch is the Senior in .22", followed closely by the Junior in .177" which has terrible accuracy but is really tiny and pocketable, then lastly the Tempest. The Tempest is just a bit too bulky for me to feel it's a convenient pocket pistol. And the plastic and aluminum don't offer the same appeal as its all metal ancestors. The HW looks significantly larger, so it doesn't appeal for the same reason, but of course everyone has different tastes. BB Pelletier wrote a four part reviewing the HW70 a few years back for pyramid. Might be worth looking into that. Here's part 4, which includes links to the first three parts:
http://www.pyramydair.com/blog/2013/04/beeman-hw-70a-air-pistol-part-4/
^very comprehensive, and sort of echoes my opinions on the Tempest. I bought one for $165 over 25 years ago, and while it only comes out a couple of times a year now, it's a great pistol. Knowing what I know now, I would have opted for a .22 cal. one, but mine has done everything it's been asked of in .177 just fine. Still looks almost new too!
For what they're worth, I'd get a tempest, shoot the heck out of it..then decide if a target pistol is your thing. I rarely punch paper with mine so I can't remember what (if I even tried) "groupings" it was capable of. I've dispatched a few pests with it, punished countless tin cans and back in the day, used to swat glass bottles at a local, small-town dump. lol Back then, we'd seek out pellets called "Prometheus" (plastic skirt, steel tip) to get the ball rolling on hefty glass bottles then chip away with them using lead. We had a system.
Might consider one in .22 if I ever find one.