TT set up vs flat

Personally I prefer to use alloy braking surface wheels if it's wet. That's about it.

It seems I've done more 'hilly' TTs than fast flat ones in recent months (probably due to a change in location) and really I kept all the equipment the same.
A lot of TT'ers will call any course with more than very small bumps a hilly course, so it really depends on the course itself. I prefer to use the same set up for both- the marginal gains from the aerodynamics of the bike and kit work at any speed, although it is marginal at lower speeds, plus my 'aero' wheels are also my stiffest, which is what I want for climbing. The pursuit bars and general feel of the bike is much more comfortable for sustained climbs under strain.

The skin suit and helmet come into their own on the descents and flats, and as long as it isn't a really hot day I don't overheat in it.
The lumpiest TT I've done is a 25 with roughly 450-500m of climb, but there was only one big hill in the middle (with a couple of ramps to the summit) and the rest just rolling where I could keep an OK speed up and the TT bike helped a lot on the flatter/ more downhill return leg.

Then there was the 15miler I rode today which people were saying was lumpy, with barely 150m of climb with only one significant ramp- the rest nearly imperceptible and ideally suited to TT machines. I was on a road bike (without clip ons), although I thought I did a good ride I was well behind most on their TT machines and knew I was considerably slower than I could do on the TT rig.

The only part of my kit I would realistically consider changing is the gearing, if there are lots of steep climbs I would consider putting a smaller gear on to help spin up them without going too far into the red- you'll be able to tell pretty quickly if you're able to do a reccy ride of the course beforehand. If you have heavy aero wheels you could think about swapping for a lighter pair to help up the hills and the acceleration over the tops and over small lumps etc without blowing up, but I don't have any really deep or heavy wheels to judge it with.

I agree with Maryka and iron-clover - for me it would have to be extremely hilly with very little opportunity for riding down on tribars to justify changing to a road bike. I still hold the women's record in the Beacon Little Mountain TT, one of the classic hilly TTs, from 2007 when I rode the whole thing on a full TT set up.

To answer ozzyosbourn - to me the crucial thing is weight. My disc was my lightest back wheel then - so on a hilly course I wouldn't have chosen to use a different wheel. Are your 808 or 404 substantially lighter than your disc? If so, then they could be a better choice but so much depends on how hilly and how much of the course is aero-friendly.

Ruth