Which rear mech - for a smoother change when climbing?
 

The rear mech on my wife's Giant Defy doesn't change very well when on a steep climb - it's kind of hunting around to decide where it wants to settle. It's a Tiagra groupset.

I know this will be less of a problem the more experienced she gets at shifting in advance, but even so, I was out on it today and got my gear selection wrong on a climb that was new to me.

Would simply changing the rear mech to 105 or Ultegra (or SRAM?) give a more positive shift? Or do I also have to buy new shifters?

(Shifters might be a daft question, but I'm no mechanic.)

Thanks.

I doubt a new derailleur will make a noticeable difference.

If it was my bike, I'd be checking the tune up starting with the derailleur hanger alignment. The more rar cogs you have, the more critical hanger alignment becomes.

Sometimes also, if the cable tension is off by nearly a full cog, it'll seem to work OK most of the time but will "hunt" around in some gear combinations.

Possibly 'new bike syndrome' then... everything stretching and going a little out of tune.