That would be all "CURRENT" Sabre's are cored hulls not all Sabre's. Sabre did not start coring until 1978 and even then it was an option beginning at 34 feet. Most buyers did not opt for the cored option unless they were racing. The new and current Sabre's are cored with Divinycell and vacuum bagged in both the decks and the hull.
If you're in Maine then the Sabre will give you a better re-sale being 30 feet and also a Sabre but the Tartan with the Yanmar would be the safer bet.
Volvo's SUCK and parts are very hard to get especially when you really need them. I was out of commission for an entire sailing season because of a stupid $5.00 part that was back ordered! I looked at 50+ boats last year and those fifty plus boats included NO boats with Volvo's. If the boat had a Volvo I would not even look at it that's how much I personally despise Volvo's.
The Sabre is a small 30 and the Tartan a big 28 so they should be roughly the same size and both are built well. It's a tough decision but if it were me I'd try and find a re-powered Sabre 30 or buy the Tartan. 28 footers are tough on re-sale though, regardless of how big they seem compared to a Sabre 30, so if you think you might eventually want to move up buy the 30 but not the one with the Volvo...
I'm guessing your talking about the Tartan 28 at Royal River Boat Yard. That boat has been cared for fairly well as I see the guy there each spring working his butt off. Other boat to consider would be a Pearson 303, Catalina 30, Cal 28 MKII, Sabre 28, J30 etc. etc..
The gray J30 listed by Jeff at East Coast Yacht Sales has a brand new Yanmar in 2005 and those boats sail great and are quite roomy for the price..
This link is a Yacht World search dialed in for just Maine boats from 28 to 30 feet and priced to 60k..