At present, the Okehampton-Tavistock route is only being discussed as an alternative route not as the main route. The ideal solution would be a tunnel linking Exeter to Newton Abbot but this proposal may turn out to be far more expensive than reinstating the Dartmoor route or any other proposed alternative.
Quite, which is perhaps why a tunnel linking Exeter to Newton Abbot doesn't apear to be on the NR list of options as provided on the link below:
To add further detail to that, the BBC have now linked to a NR paper on the Dawlish problem, and future options etc, and it includes a breakdown of that £31m into a number of individual SW resilience project costs, of which
Cowley Bridge is shown as £6.5m - £12.9m. Eight other projects are included in the total, for example Hinksey track raising at £5m, and earthworks strengthening at Honiton and Crewkerne at £3m.
Info on page 10 of this:
http://news.bbc.co.uk/1/shared/bsp/hi/pdfs/26_02_14_dawlish_jmo.pdf
In fact of all the NR options none would seam to provide much in the way of a speed advantage over the existing line and it is likely that only the route through Okehampton would generate anything of note in the way of extra passengers.
Of the routes lister I would guess that few if any would be the ideal which supporters of a dedicated Dawlish Avoiding Line (DAL) would be wanting. As it may speed up journey times but I doubt it would be by as much as 10 minutes (i.e. halfing the current journey time for the expresses which take 18-21 minutes).
If the jorney time improvementes where four or five minutes on the new DAL then that is unlikley to make a big impact on the econmey (given that there are a lot of lesure travellers for whom time saving is less of an issue).
Conversley, the route through Okehampton could have a significant impact on the local economey. For instance if it were possible to retain a cycle route as well as the rail route through Okehampton people could catch a train from Exeter and Plymouth to Okehampton to hire a bike. It could even provide for new tourist opertunities, for instance people cycleing between two stations on the line as even if they had their own cycles they are likely to want refreshments.
As even a small amount of money spent in one local business can pass through several local business before it leaves a region. For instance a cyclists buys a cup of tea and a cake in a cafe (£4), the cafe owner then uses some of that money to pay a local company for the produce that they had to supplied (e.g. cake and milk), that supplier then uses some of that money to pay the local farmer(s) and businesses who supplies them, who then in tern have people who supply them. In addition any money used to pay staff (in the first or any subsiquent business) then is likely to also pass though other local business. By the time that £4 leaves the local area (through taxes or paying non local business) it could well be that it has been accounted as significantly more than that by the business which it has passed through their books.