Tier 2 vs. Tier 5, and expiration of sponsorship certificate

I will be in the UK for five months (January-May 2012) as a business visitor, and then -- in an entirely unrelated situation -- I will be an academic exchange visitor in the UK for 19 months (June 2012-December 2013) after that.

That brings me to two questions:

1. I would be eligible for either a tier 2 (general) or a tier 5 (academic exchange) visa (because it's a "named" fellowship my sponsor wouldn't have to go through the advertising requirements normally required for tier-2 sponsorship). Since my fellowship is for just 19 months, the 24-month time-limit on tier 5 visas is not an issue. Am I correct that the only real difference for me is that, on the one hand, the Tier 2 visa costs more but that, on the other hand, the 19 months would count toward permanent residency if I end up getting a job in the UK after the fellowship ends? Am I missing any other reason why one visa might be preferable to the other?

2. I'd like to avoid the expense of flying myself and my family back to Canada this Summer for the sole purpose of applying for the tier 2/5 visa. I'm considering urging my sponsor to get a certificate as soon as possible, even though the start-date on the certificate would be 1 June, so that I can apply for (and receive) the tier 2/5 visa before leaving Toronto this Winter. Then, when entering the UK in December, I'd have the tier 2/5 visa in my passport but I'd tell the border authority that I'm not using that visa (which, of course, wouldn't be active until June). When June comes around (or mid-May, since one can begin using a tier 2/5 visa up to 14 days before the job begins), we could take a quick flight to Ireland (or train to France, or whatever) and re-enter on my tier 2/5 visa. Would this work? The Border Authority website says that sponsorship certificates expire if not "used" within three months, but I'm assuming that "using" here means "used to apply for a visa" not "used in a workplace."

You can go to the ukba website and do a points calulator to see if you qualify. I don't know what would be the best option though - you cannot enter the UK prior to the start of any visa. The sponsorship is for your employer to generally figure out - they get their lawyers involved and figure it out from there. Have a talk with either the employer or an immigration lawyer. I think they'd be able to point you in the right direction.