Haven't gone ahead yet, but are in the final stages of signing with TTB. The BT Cloud Voice and Cloud Phone systems are quite different, although I can't remember how and BT Local Business didn't exactly explain it very well.
I agree regarding BT prices - the base price isn't too bad, but there are tons of extras, even for basic stuff, which are all chargeable. On our Cloud Voice quote, the handsets were not priced separately, hence after the contract runs out, and you've paid off the phones, I presume the price doesn't magically decrease as the handsets are "free" with the subscription. We were offered the BT system on a 5 year contract, by BT (Local Business). As there is so little information on the features available for free, no doubt you'll end up having to buy add-ons for features that you need that turn out not to be included (I mean 50p per month for each phone/user for busy lamps is just silly, and I dread to think what other hidden costs there may be). BT's quote reminds me of double glazing - loads of "discounts" listed, etc.
TTB on the other hand have a more transparent pricing model, although it is a shame they don't mention handset prices on the website. With TTB, you pay for so many users/licenses as per the website, then choose the handsets, which are priced separately on the quote, no setup charges and pretty much everything is included in the price, including installation, DDI numbers and porting in numbers, which BT charge for monthly. The TTB Mitel handsets seem better than the Polycom handsets that the BT systems use. TTB is on a 3 year contract, but after 3 years you own the phones and that monthly price comes off, or they offer maintenance at £2 per month per handset. I don't know if they offer shorter contracts.
With both options, you of course have to make sure the LAN-side of things is sorted yourself, which includes adding any extra cabling or PoE switches, traffic management, etc.