Pensions are a complex subject, and to be honest, no one can really give you any substantial advice without knowing exactly the details of the pension plans you are considering. I take from your comment
"a.. pension of my choice", this will not be an employer defined benefit, final salary pension.
This is the latest thread discussing pensions (although it starts out by discussing FBAR). Nun does supply some interesting links which delve into the complexities of one type of pension. The OP is resident in the US, but whether you're in the UK or the US makes no difference if you're a US Person. The IRS views any UK pension as a 'foreign' pension.
http://talk.uk-yankee.com/index.php?topic=78570.0Again, this is only one particular type of UK pension, and each have varying considerations.
Here's another general discussion. And as a general warning, I would suggest you read any discussion concerning pensions as terms for further research, and not as fact.
http://talk.uk-yankee.com/index.php?topic=75613.0Make sure your tax adviser is conversant with PFICs, foreign trusts, FBAR, Form 8938, Form 8833, and the US/UK double tax treaty as the applicability of each must be considered. Having two pensions from two different countries is not an unusual occurrence for expats. The pensions themselves function (and pay out) well. It's only the IRS (and US Treasury) reporting for a US Person that is the challenge.
You may want to find the pension(s) you feel suits your situation best and then run those by your tax adviser for comments / potential problems. Also, be aware that professional tax advisers themselves don't always agree on the 'correct' method of reporting, given the various pension details.