Some brief comments...

If you're looking for an affordable entry point, you can't go wrong with the Operation Ice Storm box (which includes two forces of about 125-150pts ish). Even if you're not interested in the Nomads you can trade them later, and most of the included PanO troops will be useful for some builds here and there. You also get good, clean starter rules, lots of card tokens (very useful) and - most importantly - a lot of buildings, and Infinity needs a butt ton of buildings; from a 40K point of view, five or six times the amount of terrain you're used to using in a smaller space. Seriously... don't get that wrong; Infinity plays like ass on really open boards; it's an urban battle game that gets better the more dense and vertical your terrain is.

However, neither of the starter sets you list are bad at all; they are solid starting points. Just be aware that they are old, so some of the models (the Bolts and pretty much everything in the Imp box) look a bit small compared to the newer models. It's not a huge difference and it doesn't bother me, but it might bother you, so beware.

Remember that the starting boxes are starting boxes, not complete armies. I forget how many points they include, but it's probably around 150-200. Great for starting out, but not a complete or totally sensible force by any measure. You'll need other models to beef them up, but not immediately. With Infinity, start slow (150pts), work your way up; if you jump in half cocked at 300pts, it just gets confusing and complicated.

Good news though! Proxying is totally normal in Infinity, especially when you're starting out. Whatever you get, don't be afraid to proxy a few things until you decide whether you want to buy the models. Usually, no one minds at all and if they do, then they are a douchebag. Obviously, do buy the models when you can, because they are cool.

From a 40K point of view, you can build an army for less than $150 in Infinity easy, and that army will be competitive. You have to work pretty hard to make a bad list in Infinity, really, as each model has worth somewhere - you just have to know how to use it. It's a much, much cheaper game. But way more addictive... I've been playing about a year and I already have three armies, with about 500pts worth of models each. Oops.

In terms of playing the game, at the most basic level, it's a lot faster. There are some complex rules - like hacking - but once you learn your models, you can do that stuff with little fuss. Just take your time and learn one model at a time though. In terms of 40K where a 2000pt battle might take a few hours to setup and play, you can probably squeeze three or four Infinity games into that same timeframe once you get the hang of it. For me, that's the most appealing part of Infinity.

Because of the lower cost point and speed of play, I think the community is a lot less salty than 40K, but it's also a lot smaller, so it's not a fair direct comparison.