Well, I don't think anyone owns both the Sportage and Tucson at the same time, so a fair comparison may not be impossible.
Recently, just for fun, I went to see both vehicles at the showrooms (free test drive, free petrol mah!

). Unfortunately, I didn't end up test-driving any....
You may know by now that Hyundai owns Kia, so they're both essentially the same company. These vehicles were also designed on the same platform....same engine/performance specs, almost the same weight, almost the same kerb weights, etc....even the same dashboard design concept - the gear shift looked about the same too.
In fact, even the gear ratios are exactly the same for both vehicles!
ExteriorI would give this to Kia, hands down (personal preference).
InteriorKia's test drive car was a disgrace. The interior wasn't well maintained and felt really cheap and plasticky. While I'm sure the actual vehicles sold are better, the test drive vehicle really made me didn't even wanna sit in there for long.
Tucson won hands down in this aspect, with its interior looking plush and classy (although I didn't test drive it - just wanted to talk with the salesgirl lah - she pulled me in while I walked past her shop, so what can I do? Entertain her lor....).
Both as just as spacious, with the kinds of seating configuration which can be pushed down, pushed foward/backward, etc, to create more cargo space, etc.
Fuel ConsumptionBoth don't do very well in this aspect....and you have to be prepared for this. I heard this from owners themselves.
Maintenance CostsOne thing you have to know about Korean vehicles is that while the purchase price is not high, maintenance can be high. For example, Hyundai recommends that in Singapore, their timing belts get changed every 60,000 km (this is the shortest I know of as most other vehicles need changing only at around 100,000 km). Thats a very inconvenient and expensive operation every 1-2 years (for my driving style).
Kia puts their timing belt change at every 80,000 km.
Final Build QualityBoth can do lah....but expect a few teething problems if you're unlucky (eg. water condensation wetting the whole floor, power windows failing, etc). These are not very common....and not very serious. Can be easily solved. Really depends on luck, but should be OK.
Ride Comfort and QualityDon't know because I didn't test drive any. I suspect both are about the same, but there have been reports that the Sportage is more stable and firm.
PowerIf your driving style is darting off as the lights turns green, tailgating, speeding up slopes, conering fast, etc....then none of these vehicles are suitable for you. In other words, these are really not sports cars.
However, I do know a guy (my next door neighbour at the office) with a Tucson who climbs Genting with up to 4 adults in it with no problems. He says he shifts to manual and puts it at "2".....and there is some sluggishness going up, but in the end, you'll still get there. Aiyah....that guy....as long can gamble, everything else can be overlooked lah.
To be fair, he said his former vehicle (Honda CRV) also had problems climbing up Genting, although it felt less sluggish than the Tucson.
Sales TrendsIf I'm not mistaken, Kia sells less than 200 Sportages per year in Singapore. Hyundai sells around 2000 Tucsons per year. This may give you a clue about the duration to reach a sale, and their second hand values (based on popularity/market demand), should you want to sell it later. May also give you a clue about the availability of accessories for modifications, just in case you're into that.
PriceBoth are around the same price.
After Sales ServiceKia (C&C) has a better reputation in this aspect.
There have been some complaints regarding Komoco's after-sales service.
But then again, who goes back to agents for servicing after the warranty period anyway?
Further First-Hand InformationThere is a group of Tucson enthusiasts you can join - they have gatherings, go for drives to Malaysia, etc. Seems to be a very friendly group of people. They can be found here:
http://www.toc.org.sg/I don't know of a similar enthusiast group for Kia.