These are both steel tandems so fatigue shouldn't be an issue.
As for quality, Santana is just better at marketing; overall quality at the respective price points is very consistent across all brands of production tandems and factory customer service is also quite good across all brands. The dealer-end of the transaction is where most problems occur as some dealers are tandem-savvy while others are not.
As for the differences since '84, frame weight would be a major difference as the tubesets in '91 would have been more refined and a bit lighter and then there is the issue of rear spacing. Has the '84 Santana had it's rear drop-outs respaced to accommodate a larger cassette? If so, how much were they spread, how was it done (cold-set), who did it? Both of these tandems will be fairly short in the stoker compartment, the '84 Santana probably being the shortest of the two given its age.
Of course, the biggest issue is, which of the two tandems would fit you the best and how do they ride? The key is, you'll need to test ride a few tandems before you can form an opinion as to what constitutes 'better to you' when comparing different tandems. At this point, taking a single test ride on and then buying the first tandem you encounter is like getting married after the first date you ever had (not that there's anything wrong with that). Seriously, you CAN test ride a single tandem as your first tandem and make an objective decision that it is "good enough". However, bear in mind that it's just your first tandem; a trainer if you will. I think that's the track you're on and that's a good one. But, make sure that whatever you buy: a) fits properly, b) had good tires and a well-maintained drivetrain, c) shifts and handles well, and d) looks good to your partner and is acceptable to you.
As for the price, these bikes are beyond fully depreciated so the asking price is highly subjective. I would think $600 for the Burley or the Santana would be reasonable offers and negotiate to $700: they are OLD tandems. The newer (brand new and used) models they are competing with are significantly lighter, use 9 speed drivetrains, have integrated shifting, better brakes, lighter/stronger/better wheelsets, and better re-sale values. If you'd like to run some used tandem prices against a fair market value tool I've created one that you can access here:
http://home.att.net/~thetandemlink/usedhome.html
Can't tell you what to do about upright vs. 'bent. I have not reached a point in my cycling life where a 'bent is an attractive option beyond the techno-geek factor; I love bicycle technology.
Regarding new tandem prices, while there are new tandems that sell for $5k there are also quite a few exceptional models that sell for under $2k. You can also put yourself on top-of-the-line Trek & Cannondale road tandems for $3,300. Here is a link to a post where I provided a few of the entry level tandem brand/models & pricing:
http://www.bikeforums.net/showpost.p...00&postcount=4
Since you mentioned Santana, here is their MSRP line-up (Note: Co-Motion's pricing is very similar and covers the same price points):
Best Value Chromoly: $2,995
Premium Chromoly: $3,895
Best Value Aluminum: $3,695
Premium Aluminum: $4,595
Competition Chomoly $5,495
Competition Alum: $5,995
Burley is the value-leader in tandems.
Entry level chromoly/Rumba: $1,699
Entry level Alum/Tamburello: $1,999
Mid-level chromoly/Duet: $2,449
Mid-level Alum/Tosa: $2,449
High-end chromoly/Paso Doble: $4,099
High-end Alum/Rivazza: $4,099
Just some food for thought. By all means, if you've got the tandem bug act on it now. If tandeming becomes "your thing" you'll easily amortize your investment many times over relative to the quality time spent together. Moreover, if you consider what else you might acquire for recreational purposes instead of a tandem, e.g., boat, RV, trips to Home Depot each weekend, the tandem can really end up being a bargain.