Yet another clipless pedal question

The strap broke on one of my toe clips last weekend, which I am interpreting as a sign from above that it is time to go clipless.

I'm not a great cyclist, but I'm working on getting better.  I have a Cannondale C3 road bike (cost about $650) and I'm training to do sprint tris this year and olys next year.  I put about 35-55 miles per week on the bike, and my longest ride has been about 40 miles.   I'm definitely a tri n00b.

However, I also like to ride with my young kids.  They make multiple stops and occasionally disembark from their bikes to explore something of interest.  So I've been looking for a clipless pedal that also has the "option" of being ridden with tennis shoes or sandals in non-clipless fashion.

The LBS recommended I purchase the Shimano PD-M324 (http://bike.shimano.com/publish/content/global_cycle/en/us/index/products/pedals/mountain/product.-code-PD-M324.-type-pd_mountain.html).  He said this is a mountain bike pedal, but that it's perfectly adequate for the distances I'm going and for the tris.  In addition, he said that the shoes are easier to walk in, which might help when riding with the kids.  I also noticed the PD-A530 (http://bike.shimano.com/publish/content/global_cycle/en/us/index/products/pedals/road/product.-code-PD-A530.-type-pd_road.html), which the Shimano site says is a road touring pedal.

He showed me the Keo line for comparison.  The M324 is definitely heavier than even the entry level Keo. 

Question:  Given where I am on the bike, will going with one of these pedals have anything more than a negligible impact on performance or distance compared to the regular road bike clipless pedals?  Are there any other products that are better that allow the bike to be ridden with normal shoes as well as bike shoes?

Thanks!