3RZ-FE is +54 horsepower and +48 ft-lbs torque over the 22R. Also expect to make 5~7 MPG more. I make 20-21 highway MPG on 37" MTRs w/5.29s

There are better options for more power, such as a 3.4 Toy 6cyl (about +95 HP & +90 TQ) or a 4.3 GM 6cyl (about +60 HP & +100 TQ) but neither will net 20/21 MPG at 65/70 MPH on 37s.

It comes down to personal preferences so here are mine:

• I prefer how inline 4cylinders lug down and idle while Crawling without stalling. I'm talking 200 to 300 RPM without stopping. You own a 22R so you know how good it is at this. The 3RZ is just as dependable under 500 RPM. Every 6cyl I've wheeled (only experienced V-arranged 6cyls) seems to cutout around 500 RPM but I am no 6cyl expert on this, it's just that every Toyota 4cyl I've Rock Crawled doesn't mind lugging waay down.
• I prefer the lighter weight of a 4 cyl engine and transmission
• I didn't have to upgrade to a heavier rated front suspension
• I didn't have to upgrade to bigger brakes (optional but more advisable for a 6cyl swap)
• I have enough torque but not so much that I am blowing up parts every weekend
• 4cyl provides more free space under the hood for auxiliaries like on-board-air, welder, while retaining A/C, etc
• Less heat generated so you don't have to run a larger radiator
• Only one header instead of two, more free space encourages air flow
• Lighter exhaust system
• Cheaper tune-ups:
  
  • Fewer spark plugs/wires
  • Less engine oil
  • Less transmission gear oil
  • Half price valve jobs
  • Less injectors to get cleaned
  • Less expensive clutch kits
  • Aftermarket headers, header powder coating services are cheaper
  • et cetera...
• 3RZ uses a timing chain instead of a timing belt like the 3.4 6cyl (heavier, nosier but more dependable)
• Hmmmm.......
• Tires will last longer (less weight, less likely to be doing burnouts at every green light)
• Suspension will last longer
• Motor and transfer case mounts will last longer
• Less likely for the frame to develop cracks (big issue with 8cyl swaps)
• The 3.4 is much taller than the 2.7, and will require hood modifications or a body lift
• Easier to remove / reinstall engine - lighter, less things going on, less to disconnect/reconnect
• Hmmmmmmmmm......................
• Less things to go wrong when you're at 10,000 feet elevation in the middle of nowhere
• If you are on an extreme side-hill angle then you won't have a bank of cylinders swimming in and cooking engine oil (V6 vs inline arrangement, however the RZ-family does have a slight tilt to them)
• Less moving parts... Fewer things to wear out... Ok now I'm reaching pretty far

Downside is that the 50% power increase won't be the kind of blow-your-socks-off power that a 3.4 or 4.3 6cyl will provide. If you are "really" looking for more power then you will be disappointed with another 4cylinder unless its boosted.

Which brings me to the "Middle Ground" which would be to Supercharge the 3RZ but the smog legal electronic solutions available leave much to be desired. As do the associated costs of the S/C, higher grade fuel requirements and reduced fuel mileage. I was S/C but it never ran right so I sold it. I regret this. Next year I plan to start looking for another S/C and will go Mega or Micro-squirt and finally do it right with a dyno-tune.

Preference. Some can live with *having* to pull steep grades in 4th gear (= 2.7 owners with 37" tires) while others desire to stay in 5th to show off (= 3.4 owners with 37" tires). I discussed S/C'ing my 3RZ-FE again which means the following: When there is sand, mud, or snow I do wish I had just that little bit more torque. Fortunately for me I am not into sand or mud but I do enjoy snow. Others want more. They should go with a 6cyl and leave the how-did-you-do-that-so-smoothly finesse Rock Crawling ability to us 4cyl guys

Regards,
BigMike