Ok, it is human nature to always want to save your hard earned money, but it almost always comes back to bite you in the butt when you "go cheap" on something you know you need. Is ice shield necessary? it could be by code, so look into that and contact your local building department. I think it is absolutely essential. It "forgives" alot of mistakes and adds another layer of redundancy to your roof in the problem areas.
In my area, Chicago, Ice shield is requried at the gutter lines / eaves. It should start at the gutter / edge of the roof and extend not less than 24" past the interior warm wall. It is my understanding this is pretty much universal across most of the country. I also like to isntall ice shield in the valleys as well as any pre flashing areas. These pre flashing areas include where the rofo meets the wall, around chimneys, skylights, pipes, any area that will later receive metal first gets pre flashed with ice shield. It's the $100 difference between a roof that leaks in 5 years or in 25 years in my honest opinion. I have been doing this pre flashing now for about 10 years and haven't had one leak in the areas I've properly pre flashed.
Reuse vents? IMMEDIATELY DISQUALIFY THAT PROPOSAL. HE DOES NOT KNOW WHAT HE IS DOING!!!! Is that clear? So let's assume for a moment that your ventilation is under sized. It usually is undersized in my opinion because depending on when your house was built the codes were likely different and honestly people just didn't ventilate like the should have when energy was cheap... So reusing the existing ventilation might be a no no. Plus do you want a new roof with old vents?! Does that make any sense to you? Again a couple hundred more bucks to do it right.
Will he also reuse your old flashings? Hog wash I say!
Is ridge vent right for your roof? Maybe, maybe not. It depends on the architecture of your roof. For example on hip roofs ridge vent is usually too small. There are specific formulas we professional roofers use when calculating how much ventilation is needed. Once the calculation is performed we can determine which type of ventilation will achieve the best results.
You've got two quotes reasonably close to one another, and you've got one just way cheaper. Ask yourself why that is. Something smells fishy and I think you already knew that or wouldn't be asking us. What other corners is he cutting?
Read these two lins for a massive eduation on roofing and working with a professional roofer: http://reliableamerican.us/services/shingles.htm