As for 15mm figures, these days, you can't throw a rock without hitting a descent range of 15mm Sci Fi figures, they're literally all over the place. It mostly depends on your budget and possibly location (North America, EU, Antarctica..etc...)
When I look at a range of figures, I look at the depth of the range. Does the range include support weapons, are those support weapons, heavy, light or a mix of the two. Is there enough poses in the range for some creativity and uniqueness(varied poses, command..etc..)? Do they have a varied range of weapons (rifles, saws, GL, RL..etc...)? Also, do you want a one-stop-shop..ie...does the company carry figures, terrain, and vehicles or do you prefer shopping around and ordering from multiple companies?
Out of the two mentioned, I'd go with Rebel Minis Earth Force because of my above stated criteria(Mike's figures are nice and he's great to deal with). Nothing against Blue Moon, they're outstanding figures, but they don't offer the depth I want in their figure ranges.
Personally, I use Ground Zero Games, they're reasonably priced, but I usually order when Jon (Mr. GZG himself) runs a sale or "special offer". Jon is also well known for his outstanding customer service. I live in the states and I don't hesitate to buy from him (in the UK) even with exchange rate/shipping charges.
Some others to consider in no particular order:
ClearHorizon Miniatures - USA, Harold is awesome to deal with, his castings are awesome....I use his figures for SpecOps type teams
Khurasan Miniatures - USA, outstanding miniatures, odd to deal with at times though, but his ranges usually have a lot of depth and you can't complain about his variety...
Brigade Models - UK, I think they acquired Army's Armies 15mm range of near-future Sci-Fi figures, great guy to deal with, lots of options to choose from including a TON of vehicles!
White Dragon(?) - Not sure about them, nice looking figures, not sure of their availability
Critical Mass Games - UK - defunct business...but I think the range was sold to another company, not sure who at the moment, shame though, they had a great range of figures and vehicles
Acheson Creations - USA - I *think* they're going to carry some 15mm THW Star Army figures that they produced during the recent Into the Tunnels KickStarter, but Ed would know more about them and they're products probably?
Old Crow - UK - Not sure he's still in the business, but he has a great range of vehicles, I think he produces "on demand" rather than running a shop
Highlander Studios - USA - I think he has a smaller range of "character" type sci-fi figures.
QRF - UK - Sci-Fi range of figures and "xenos", also carries a lot of historical 15mm.
Peter Pig - UK - mostly 15mm historical...I'm not sure if they carry any 15mm sci fi or not, so that might not be helpful!
I'm sure I've missed some, but you hopefully get the idea, just food for thought. I hope that helps!