Speaking as a low-hours PPL who has done a bit of both, i'd do the aeros. Great fun, great for your confidence and great for your stick and rudder skills.
The IMC is alright, but it's not really very exciting. An IMC and some occasional practice will give you a few options that aren't there for the plain-vanilla PPL, but really using the rating to go places in cr@ppy weather takes a lot more time and money than the 15-hour it takes to get through the test - and if you've got the kit, the money and a desire to really get places whatever the wx a FAA IR makes more sense for not much more money.