Hi Jonathon.
I am assuming that your new boiler will be a combi, based on the models you mention? Is the existing floor standing boiler a combi, or is it heating the hot water via a cylinder?
I'm a bit confused by the advice you mentioned - the engineer told you to avoid Baxi, but to look at a Duo-tec or a Logic+ ?
The Duo-tec is a Baxi!
Its possible that he was advising you to avoid the newer Baxi Duotec GA, and to try to get the older Duotec 1, which had a good reputation. You can still get Duo-tec 1, but you might have to shop around. The Potterton titanium is essentially the same boiler.
The Logic + is a good recommend - well thought of by forum members, and probably the biggest boiler success story of the last few years. So long as you specify the "plus" part, you will get a 5 year warranty.
The market leaders are Worcester, followed by Vaillant. Both makes have their fans and their critics, and no doubt you will get plenty of comments from both sides. Glowworm is made by Vaillant, but they are generally a bit cheaper.
There is a bit of a warranty arms-race going on with manufacturers at the moment, many of whom are either putting out blanket warranty upgrades, or upgrades if fitted by one their "approved installers". You should be able to get 5 years from most manufacturers on at least some models.
Main is another brand worth looking at - its a stable-mate of the Baxi, decent quality and quite cheap. On the downside, you will have to pay a £99 upgrade to get 5 years warranty.
I hope this helps.
Ray