Few things that are important to consider:
1. Are you looking to go in to academia or pvt practice? Choose programs accordingly. For academia u may need to publish and publish! However The distinction between academia and pvt practice is disappearing fast .. as an echocardiographer at a major center I read 40-60 studies on a daily basis! And no, these are not pre-read studies!
2. Advanced subspecialty fellowships entail closer interactions with fewer faculty than general fellowships. Hence it can be harder to 'shine'.. familiarity can breed contempt. What is the culture of the lab like? In most cases its set by the director and key faculty.
3. Not all programs will give you great structural or ACHD experience..may need to ask beforehand. For example, how much fellows are involved in structural cases? Not all big labs do strain, RV stuff well either. For those interested in academia, its important to see how much exposure there is to cutting edge stuff like new equipment and softwares etc.
Look at the publishing record of fellows.
The fellowships that stand out:
1. Mayo- great for echo.. probably the best lab in country at the moment.. but its 'very subsubspecialized'- wd need to see how strong the 3d, structural and adult congenital echo experience is for all level 3 fellows. They publish a lot!
2. Columbia- Becky Hahn is at the helm- its the structural powerhouse. Dont know how the mentorship is.
3. MGH- Picard et al. Strong lab. Lot of mitral valve stuff. Structural volume low.
4. University of Chicago- Lang et al. Very strong 3D.
5. UCSF- Foster/Schiller et al. Very strong adult congenital experience.
6. CCF- very strong globally
Other prominent programs I can think of:
Methodist- lot of big names here
Duke
UAB
Emory
Penn
Northwstern